Normally, you can never split a 40A supply to serve two loads rated for a 20A circuit. Except here.
This is an allowed exception for supplying oven/range loads. It's in NEC 220.55, referring to Table 220.55, Note 4:
The branch-circuit load for a counter-mounted cooking unit and not more than two wall-mounted ovens, all supplied from a single branch circuit and located in the same room, shall be calculated by adding the nameplate rating of the individual appliances and treating this total as equivalent to one range.
NEC 220.55 and Table 220.55 also includes some derating (or more accurately, permissive up-rating) that overrides the normal "125% for continuous" derate found in NEC 210.19(A)(1). It appears to be permissive. Speaking of 210.19, ThreePhaseEel points out
210.19(A)(3) Household Ranges and Cooking Appliances. Branch- circuit conductors supplying household ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances shall have an ampacity not less than the rating of the branch circuit and not less than the maximum load to be served. For ranges of 8 3/4 kW or more rating, the minimum branch-circuit rating shall be 40 amperes.
Exception No. 1: Conductors tapped from a 50-ampere branch circuit
supplying electric ranges, wall-mounted electric ovens, and counter-mounted electric cooking units shall have an ampacity of not less than
20 amperes and shall be sufficient for the load to be served. These tap
conductors include any conductors that are a part of the leads supplied
with the appliance that are smaller than the branch-circuit conductors.
The taps shall not be longer than necessary for servicing the appliance.
Exception No. 2: The neutral conductor of a 3-wire branch circuit
supplying a household electric range, a wall-mounted oven, or a
counter-mounted cooking unit shall be permitted to be smaller than the
ungrounded conductors where the maximum demand of a range of
8¾-kW or more rating has been calculated according to Column C of
Table 220.55, but such conductor shall have an ampacity of not less
than 70 percent of the branch-circuit rating and shall not be smaller
than 10 AWG.
This also overrides the 125% rating by saying a 40A breaker can definitely supply two 20A ovens, and saying a 8.75KW-9.6KW oven is allowed on a 40A circuit.
The 225A welder would draw 50A @ 240V on it's maximum setting. The 17,000 BTU/hr heater is electrically equivalent to 5kW, so about 21A @ 240V. If you want to operate both simultaneously plus lights (not much power required for those, however), you need a 100A subfeed to your garage.. 60A will not cut it.
A 100A subfeed to the garage requires 3/3 AWG NMD if the garage is attached and the cable is protected and not in a wet location. If the garage is separate, then you're looking at running conduit, with 3 AWG RWU90 line and neutral conductors, (minimum 6 AWG neutral conductor if your locality lets you run a reduced neutral), and a 4 AWG ground.
The breaker would be 100A, unless you can find one that's larger than 70A but less than 100 A. You will likely want to upgrade your service to 200A (Imagine someone using the stove and electric clothes dryer while you weld in your heated garage…) In my jurisdiction, a load calculation would result in a 200A service being required.
Best Answer
"Sylvania" had a couple of different types of breaker panel systems during their forray into that business. One of them was called "Zinsco", the other was "Bryant" panel and breaker system. The Bryant version was eventually taken over by Eaton / Cutler hammer and is now called their "BR" Series (BR for Bryant). Zinsco breakers were / are a disaster waiting to happen and in the opinions of most people, should be ripped out and replaced as they had a tendency to not work and/or catch on fire. Zinsco as a stand-alone company went bankrupt ages ago because of this and Sylvania was unable to rescue them, so they too got out of the business. There are some aftermarket companies selling redesigned versions of the Zinsco breakers, but before wasting money on them, check to make sure your homeowner's insurance policy doesn't have a rider saying that they will not cover any losses from failures of Zinsco (and Federal Pacific Stab-lok) breakers. Some do, and in some jurisdictions you cannot make any alterations or repairs to these panels, you must rip and replace.