350 heads will bolt up physically, but you need to be aware of different combustion chamber sizes. Your 307 probably has ~70cc chambers.
Large-chamber (76cc) 350 heads will drop your compression ratio, which means power goes down a little, fuel consumption goes up a little, and you won't be able to realize the full performance gain from a camshaft upgrade. A camshaft upgrade with a compression ratio downgrade makes an engine weak at low RPM, and not as strong as it could be at higher RPM.
Small-chamber 350 heads (64cc), will increase your 307's compression ratio, and you might see a small power & fuel economy increase. The big payoff is when you go to a camshaft with more duration, which will raise the point in the RPM range where your torque peak occurs, and you'll see a significant increase in horsepower. The higher compression ratio will keep it from losing so much low-end grunt when you use a "bigger" cam.
Make sure the heads you get have the accessory holes in the ends like your current heads have, or you'll play hell mounting your alternator, power steering, etc. If they don't have the proper holes, you can't just drill and tap them and expect them not to break. Sometimes they don't, but I wouldn't chance it. '69 & '70 300HP 350 cars (not trucks) would be the primary source for small-chamber heads with accessory holes, but '96-'99 L-31 Vortec 350 truck heads are better, although they do require a specific intake manifold.
Bullheimer's right on target with using 283 pistons and 400 rods in a 307. The only available aftermarket 307 pistons have their height reduced as much as .020", and will decrease performance and efficiency, and increase the chance of detonation if you use low-octane gas.
Last edited by jimfulco; 12-25-2008 at 08:38 PM.