That's the largest chicken I've ever heard of.
That said, 3 to 3.5 hours at 325℉ should be sufficient. Unfortunately time doesn't really matter when cooking chicken, but temperature. You should definitely be using a meat thermometer and cooking long enough for the breasts to reach 160℉ and the thighs 170℉.
If you don't have a thermometer the next best thing would be to poke the thigh when you think it's done and confirm that the juices run clear. If they are not clear, then it needs to be cooked longer.
Your best method of making chuck tender is to cook it low and slow, as per traditional barbecue.
The reason for this is that chuck is a relatively tough meat, full of connective tissue (the protein collagen, among others).
By cooking low and slow for a long time, you raise the internal temperature of the meat to about 170 F to 180 F for an extended period, at which temperatures over time, in the presence of moisture (either part of the meat itself, or external) the collagen will convert to gelatin, converting from a touch to a succulent and tender product. This is the principal employed in both braising and barbecue (as opposed to grilling, which is a high heat technique normally).
Note that the collagen to gelatin conversion creates the tenderness, it is not preserving tenderness, since it was not a tender cut to start with.
At this point, since you have already sliced the meat, I believe your best approach would be to create an indirect fire or flame. Place the meat in a disposable pan layered together to minimize drying, away from the fire, and allow to cook slowly for at a low temperature (say 250 F to 300 F air temperature as measured in the grill with its lid on) for several hours, at least about 4 hours and up to 8 to 12 hours, until it is fall apart tender.
You may also do this in the oven rather than on the grill. It will also be amenable to braising techniques, as opposed to dry roasting/barbecuing.
While it is too late for this particular chuck, in the future you may have better success not slicing it prior to barbecuing.
If you are thinking of grilling as a high temperature technique and do not want to do the low and slow methods, I am afraid this will not work out well with chuck. It is naturally not tender, and these high temperature grilling will not facilitate enhancing the tenderness. If you are time constrained, you would be better off freezing your marinated chuck for another time, and cooking something else that does respond well to the fast, intense heat of grilling, like steaks or pork chops.
Best Answer
I'd use the recipe they have on their site for pork shoulder as a guideline, since it's very similar in terms of size and shape to a beef roast (just not as heavy).
They estimate 10 minutes per pound, which is more or less consistent with the per-side estimate of most broiler recipes (which is very similar to infrared cooking).
So start with an estimate of 40-50 minutes but do what they recommend and monitor the temperature very carefully.
I have to point out that 4-5 pounds is very heavy/large for high-heat cooking methods like broiling or grilling (or infrared); it's not the same as infrared "frying" a turkey or a leg of lamb which has a very high surface area/weight ratio. Steaks would no doubt come out great this way but larger roasts respond better to roasting, so don't be too surprised if your roast starts to get seriously charred on the outside before it's sufficiently cooked on the inside.
You might want to consider starting with smaller, 2-3 lb roasts, and move up the weight next time if those come out perfectly. Based on their claims, it should take roughly the same amount of time for one large item vs. two smaller items anyway.