I like to use 3/16" so you can tap the rails for brake line clamps, fuel line clamps, etc.
You mention 20" as the width of the frame. This is actually too narrow for a 350. My frame measures 23.25" at the front of the engine and tapers to 27.5" at the cowl (inside dimensions). The mechanical fuel pump is only 1" from the frame rail and the low mounted alternator is about 1/2" from the frame rail. This is on a 27T, so the car is narrower than what you have, but you still need the space for your engine.