I have a 68 small block camaro and a 1985 big block chevy can my original frame mounts and motor mounts be modified to work with this engine, also can I use these exhaust manifolds. will i need to change oil pans. will the H.E.I clear the fire wall. using th350 with stock cross member.

No, You must change the frame and engine mounts. This will offset the engine to the passenger side approximately 1" and forward about 1/2". Your transmission crossmember will also need to be changed. The radiator, radiator shroud, a/c suitcase if equipped, and other things will need to be changed.

Will an HEI fit ? Maybe... maybe not. It depends on alot of things. I wouldn't count on it.

Exhaust manifolds... maybe. Oil pan... maybe.

I have read where most have invested $2,000 into converting a small block car to a big block. This is not including the cost of the engine or trans.

the HEI wont fit unless you fab the firewall

This is a decent article on the subject from Classic Industries:

1967-69 Small Block to Big Block Engine Swap
http://www.classicindustries.com/review/tips/tip016.htm

I'll bet the pass side will fit, as I think they were the same for all models for many many years. The Driver side? Depends on what the motor came out of, but I'll bet you'll need to find a replacement. If the exhaust dumps straight down, it will not fit. if it dumps back at 15* or so, it might.

I borrowed an HEI from someone, when I was having trouble getting my engine to fire, and it fit.....and everything is correct for 69 b/b.

An alternative would be Dave's Small Body HEI, which is a conversion to HEI that uses a stock aluminum small distributor.