Does anyone know if it's possible to swap a Lionel FT A with a Lionel F3 A shell? Have some nice Burlington shells that I would like to swap with a better running diesel chassis that has sound.
No. Completely different animals.
TRW
Well...what do you mean by "swap"? Drop-in? Nah. Modify and swap? Almost certainly; it's all just plastic and metal and tools. I have swapped diesel bodies from one brand to another (some time back).
BTW, in the real world, the EMD FT was a shorter locomotive than the later F3/F7/etc., I do believe. So, if the models are precisely 1:48 (but there's always fudge), that can add to the issue.
I'd open them both up and eyeball the heck out of the situation.