My '69 started off as a 350 HO, but the PO swapped that for a '70 YD block, which is supposedly from a 290HP big car. I don't know what heads originally came on this engine, but it currently has a set of 1968 #15 heads, which are small valves but high compression - looks like somewhere round 72-74cc according to factory spec. (Not sure what they actually are.)

I'm planning on rebuilding the top end of the engine to be as close to stock as I can. I have a set of the correct #48 heads which I plan to have rebuilt but haven't been able to measure yet since they're just bare heads.

My understanding is that the #48s tend to be in the 68-72cc range, which isn't much less than the #15s that are on it now. I currently don't have any problems with pinging. I haven't pulled the heads so I don't know what kind of pistons are in there.

Would it make sense that if the #15s can be used that the #48s can likely be swapped on? It doesn't seem that there's a huge difference.