Can you use Oldsmobile 307 5A heads on a Oldsmobile 403?

YES, the 5A heads FROM A 307 can be installed on a 403. The307 heads are a BETERR choice than the 67 to 72 Olds 350 heads. The 307 heads have similar sized intake & exhaust ports to the stock 403 heads. The 307 heads have a much smaller combustions chamber at arounf 67cc. This alone will raise compression to around 9.5:1. The only down side is the 307 heads have much smaller valves. You really must get larger valves installed into the 307 heads. The stock 403 valves will fit with some machining (of course) and you'll have a high compression 403. Get some port work done while you are at it & a little polishing too. Buy a new intake & have it machine & port matched to the intake & a middle of the road Lunati cam. This will result in a 403 producing around 330 HP. I know it works because this is exactly what I did. Enjoy your Olds, Mark