My car is 1986 firebird with a 305 4bbl. I am currently building a 1977 350 4bolt main for a swap. I have vortec heads, 30 over pistons, new crank and rods, double roller timing chain, 1.5 rockers, and a crane 480/480 cam. My question is, is the 305 a two peice or one peice rear main seal and can I use the flex plate and balancer from my 305 on to my 350. The 77 is a two peice. Also can I use a standard 350 oil pan for the 77 block or will I have to buy a special one for the swap to make it fit in my 86? I did a search but realy didn't find what I was looking for to answer these questions. I plan on using my stock 700r4 for now. It was just rebuilt before I got the car and shifts realy hard. One of my uncles said it the best shifting 700r4 he has been in. Thanks for the help.

Any help would be great!!!

I thought that some one have replied by now. I know this swap had been done and I don't want to go into it blind.

If your old 305 had center bolt style valve covers it may be a one piece rear main seal block. If it has the old style perimeter bolt valve covers its likely a 2 peice seal block.
The balancer and flex plate will swap onto the 350. Inspect it closely for cracks between the bolt holes and broken welds on the ring gear. this would be good time to invest in a affordable new thicker aftermarket flex plate and a new balancer from Professional Products or other such company.
www.professional-products.com

Sorry, somehow this one got by me.

The '86 engine is a one-piece rear main seal. So, you will need a 153-tooth 2-piece rear main seal flexplate. I've seen good new ones for around $25.

The '86 damper (it's not acutally a "balancer") probably isn't a good idea for 2 reasons: 1) It's a 305 damper, and 2) the '86 timing tab is at 12 o'clock and the '77 is most likely in the 2 o'clock position. Better to go out and get a 350 damper.

A '77 350 4-bolt probably came out of a truck. Since the dipstick will be on the driver's side, the '86 pan won't fit. Whether the stock '77 pan fits in the 3rd gen, the answer should be "yes", but there are possible variables that can't be known simply by saying "77 350 4-bolt". As long as the basic shape is the same as your stock pan, you should be okay. The point where it starts dipping down to the sump is the key factor.
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'86 was a "bastard" year. They were perimeter bolt valve cover, flat tappet cam, one-piece rear main seal.

Thanks guys for the help. About how much will a new dampner for the 350 cost? Thanks again.