hey guys
I have an 87 firebird it had a manual 305 originaly I swapped it out for a 350 the 350 is externally balanced and it has a flex plate for an auto trans I tried to swap it with my 305's flywheel but it wont fit on the hub does any one know what flywheel will fit? that isnt too expencive.
thanks in advance

it should fit. It should be a very tight fit. maybe one or the other has an earlier crank swapped in? There are rear seal adapters to allow this to be the case. If it just barely doesn't fit, get a rubber mallet. If it dramatically doesn't fit, do some more checking.

the engines casting number is C239 but i also have a 3970010 code. I dont know if that helps no the 305's flywheel has a hole for the hub of the crank and the hub on the 305 is about 1/2" less in diameter than the 350

Any flywheel from an 82-85 305 Camaro or Firebird will fit. You have a 2pc rear main seal(RMS) engine and a 1pc RMS flywheel. You can either pick up the adapter like Atilla mentioned, or get the 2pc RMS flywheel.

Given:
That means
is not true. Typically, "externally balanced" means weight has to be added to the flexplate/flywheel in addition to the cast-in counterweight on the crank outside of the rear main seal, such as the factory 400 small block.

You have what is called a "two-piece rear main seal" 350. As stated, the early 3rd gen flywheel, even though for a 305, will fit and work just fine. I know there are adapters to put 2-piece rear main seal cranks in 1-piece rear main seal blocks, but I have never heard of an adapter to put a 1-piece rear main seal flywheel on a 2-piece rear main seal crank (nor would I want to use such a thing if it existed).

I'd suggest sticking to an '83-'85 T5 flywheel - I don't know if it's different from the 4-speed flywheel of '82-'83, but no need to take that chance.

I forgot about the 4 speed being used for those 2 years.