I have a 1991 RS with a 350 TBI in it(not original engine of course) The car starts fine when its cold, but after it gets up to operating temp. and you turn it off for a few minutes and when i got back to start it, it starts up and dies after a few seconds. What could be my problem maybe?

 

Maybe vapor lock from the fuel getting too hot? Check your fuel line routing on your replacement engine and see if it's getting too much heat from somewhere. Rule of thumb for electronics in the engine bay is that you wanna be able to hold your hand on it for a few seconds when everything's at operating temp. If it's too hot to touch, it's too hot! Don't know how it applies to fuel, but probably similar.

 

Your description sounds like it may be over-rich due to injector leakage, or possibly a weak ignition coil or HEI module that cannot function after heat soak. Does starting when hot become easier/faster if you open the throttle while cranking?