How much more horsepower would I get for putting 305 heads on my 350 with a edelbrock performer intake?

 

42.

 

42?? maybe if you correct the intake valve size difference
305 I 1.84 vs 350 1.94
I would install 1.94 1.6 valves
the compression will help from 350 76cc chamber to 305 64cc chamber
the runner length will be smaller on the 305 heads porting would be advised

 

Saabster was just being funny. Or not.
Anyway, If you want to do this, use the 081 heads. They are around 55cc, while the older 416 heads are around 58cc.
Get the 081s, then order these:
http://www.competitionproducts.com/S...ductinfo/8430/
have your local machine shop put a small 30 degree back cut on each and every one of them, then have them cut the heads to match.Be sure to specify that they use angles of 30, 45 and 60 degrees. Then have them do a bowl hog with a 75 degree cutter.
This will get you a dramatic improvement. You'll feel it instantly. But you'll forever need premium unleaded, and on really hot days you'll need octane booster.
Expect about 20 horses and about 25 ft-lbs improvement just from the head work. It'll pull to 5500 just fine with a moderate cam, something like an Elgin 1166.

 

stock form 305 heads on a 350 will stop making power over 4500. They feel good down low, but stop pulling quick. Ive done it twice. First time I was wanting power and tried to save some money. Learned from that one. Second time I was rebuilding a 90 RS. Originally a 305 with spun main bearing. Built a complete stock 350 for it, reused the 305 heads since it was a stock rebuild.

I dont recommend it.

you can do what atilla is recommending for alittle more umph. But dont expect a huge gain.

What are you really trying to do? Add alittle power, do a cheap rebuild, or are you wanting to add a lot of power?

 

42 is the answer.

 

But the mice decided that made no sense, so they concluded that 42 is how many roads a man must walk down. I did read that series.