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.

Pontiac chamber sizes have been found to generally be larger than advertised. with that said you are basically right theoretically comparing those heads as far as size. the problems arise in the real world because of variables such as actual chamber size originally and if the head has been cut to flatten it out and most likely will have to be cut again, That is why actually ccing them is a must to get a real # otherwise you can only ball park it. As far as a static # most shops shoot for 9.2 to 9.5 as safe for pump gas with iron heads and most likely you will be higher than this. I have had plenty of people running 10.5 tell me they don't have a problem running pump gas and "never hear pinging" however spark knock can be hard to hear over road noise and stout exhaust and have personally seen the devastation to chambers and rod bearings at a buddy's machine shop when things go wrong but when the customer is told of the problem they almost always answer with I never heard anything. my advice to you is if you want to use the 48's and I understand why you would I would definitely figure out my static compression ratio and go from there as far as fuel types. If you have a small airport near you mixing av gas with pump gas is the cheapest way to go. you will filth up your plugs a little more because of the much higher lead content but that's cheap insurance to safeguard your engine. A mix of race gas works to but more expensive. I don't know if you have seen this but here is a basic calculator to help you understand this and there are many others. Good luck. http://mobile.wallaceracing.com/