I have a couple of long runs too and I used a fan until I upgraded the HVAC unit. Before you get a booster you need to measure the air flow out of the two vents and compare them to other vents in the house.
How to measure air flow? Well pick up an Air Flow Meter. However I have a suggestion before you spend $$$ on this. Get a large trash bag, configure it to open about the size of your duct opening (take off cover), hold it tight on the duct opening, measure how long it takes to fill... compare several vents. Note the air should be running when starting this - since how long it takes air to come out factors very little into the cooling equation. If you are confident with the bag results and they are obvious then skip the pricy tool. ##Side Note## (A friend of mine uses one of those garden wind spinning things and measures the rotations over a given time to compare air flow. You have to have the angles/distance the same from the vent and he has to have his iphone slow wdown the frame rate to count. But damn that mcgyver-bastard, pretty sure its more accurate than my empty trash bag! But it does involve slow-motion so my solution is more practical)
I would also take temperature readings inside each duct while on after 5-10 minutes. How cold is the air you are getting?
If you are getting comparable air flow and temp you can add another duct or bigger. Really up to you.
If you aren't and I am guessing you aren't then you need to first get rid of the flex lines. They squash air flow, they are affected by outside temperatures more, and they leak. Put rigid metal ducts in. If the ducts are in the attic then you need to insulate the crap out of them.
Then if you still have issues you need to measure air flow and temp again. If air flow is poor you are looking at a booster. If it is relatively the same, add another duct.
Then you are talking about adding a return to the room and over-insulating the attic above the room.
In theory this is what HVAC professionals are supposed to do. In practice, most of them rely on antiquated rules of thumb to size equipment and use existing ductwork, even if it has major problems. A certified energy rater is who you're looking for and who will be able to definitively tell you what you need to know, but you can learn a lot yourself to be a more informed consumer.
The correct size of equipment for your home is discovered by doing a Manual J load calculation. You can do an 85% as good approximation yourself in 15 minutes here: http://www.loadcalc.net
For a 1930s house, unless major air sealing, insulation, and window replacement work has taken place, the load may be as high as 1 ton of AC per 500 square feet and 30 heating BTUs per square foot. But those would be worst-case scenario numbers; run them yourself with that web tool and get a ballpark estimate of your house's loads.
The most appropriate type of equipment for your house depends on your climate, your budget, and the fuels available. If you live in a mostly-cold climate and have natural gas, then a gas furnace makes the most sense. If you live in a mixed hot-and-cold climate but it doesn't get too cold, then a conventional heat pump may be a good bet. If it gets both very hot and very cold, then AC + a gas furnace may make the most sense if you have gas; if you don't, then a Japanese ducted mini-split or conventional heat pump is a better choice.
As for your existing ducts, it's impossible to say whether they're adequate or not without knowing anything about them. But they may be inadequate if they are in the attic, or if they are in a vented crawlspace, or if any rooms get much hotter or colder and other ones, or if a lot of air leaks out of them.
Another thing to keep in mind is that you're not really hiring someone to design your home HVAC system; your home already has an HVAC system (well, the "H" and "AC" parts, at least). If every part of it is unsalvageably wrong, expect to pay mucho dinero to completely replace everything. New equipment and new ductwork and all the demolition and drywall repair entailed by such a job will probably cost you $15k or more... possibly much more if you live in a high-labor-cost area. Consider how much you actually need to replace and how much you can simply live with, especially considering that many comfort-related deficiencies in a house's heating and cooling infrastructure can be ameliorated by improving its insulation and air sealing. There are many such opportunities in an old house, including the extremely low-hanging fruit of air-sealing the basement, weatherstripping any old windows, and adding more attic floor insulation.
In general, it's a good idea to do all this work before replacing your equipment, since if you buy correctly-sized units now, they will be too big after you perform the work. You can mitigate this to a certain extent by purchasing "modulating," "variable speed," or "two-stage" units, as they are capable of reducing their output at low-load conditions. In theory you won't have to use your furnace for another 4+ months at least, so you have some time to tackle those sorts of projects before having to replace it, which will allow you to purchase a smaller one and save money, or a better one for the same price.
Best Answer
If the addition that you mention has 3 side walls and a ceiling, heating and cooling this room from the same furnace as the rest of the house is nearly impossible. The 4 surfaces exposed to the weather will loose heat in the winter and gain heat in the summer at a faster rate than the rest of the house. If this is the way the addition was built, then a separate heating and cooling unit or the existing system should have been zoned to provide it's own temperature control. Adding too much air supply will cause large temperature swings in the addition. As far as giving advice on running the duct, I would have to be there to see the actual space and items you mention. And "no" I would not use plaster board for a duct. At this point, I would add it's own heat/cool system or better yet have a professional in your area give their advice.