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if you're looking at the block the heads came off, 4" bore = 350.
305 is a smaller bore but I'm not sure of it off hand. (better make sure the heads came stock on the block though) Otherwise get the casting numbers off the heads and there are numerous resources online to look up head casting numbers that will tell you if they are 305 or 350 heads.
Or post up the numbers and someone can look them up.
i have a pair of 305 heads that look pretty fresh and are really clean. they were taken off a motor and i was going to port and polish them but i don't have the time to do it so i'm trying to sell them as is. they are 416 heads which are the best flowing factory 305 head out there.
somebody asked this a while ago...the answer was its a bad idea beacause the valves, runners, and cc are too small. They may bump compression but I seriously doubt you'll see any performance gains.
why not get some 58cc vortechs? (I think they make 58's)