Or you could simply ask the advice of the knowledgeable guys like Ponchoshop, Banshee, 428HO, etc., instead of trying to guess yourself into an expensive non-performing motor.
Factory HP ratings had everything to do with the way the engines were tested and the type & grade of fuel used. In the 60's, high octane (98 to 105) gas was common, and engines were tested at the flywheel with no accessories bolted on. You will not get similar numbers using today's pump gas.
If you want to get any level of performance, use heads that have large intake valves. Converting from small to large will cost you as much as buying large valve castings. There's few good reasons to do this, and 1968 #15's are not one of them.
Even though some heads have identical dimensional features, such as chamber size and valve size, there were significant differences in how those heads were set up by the factory. Single springs versus dual springs; heavy duty versus standard duty; different spring heights, etc. The casting designs, in terms of port volumes and runner lengths, were nearly identical for all of the large-valve D-port heads from 1968 through 1970 (#64 being an exception). The casting number was part of a bigger picture. It applied to the set-up of the head more so than just the casting itself. #62 and #48 heads (of which there are two variants) are nearly identical except that the factory used heavy duty dual springs on the #48 and standard duty dual springs on the #62. I have also found that #62 heads have 75 cc chambers, with some as high as 80cc, versus the almost-universally consistent 72cc for #48. 350 HO Heads (also #48) usually run at about 65cc. They are impossible to distinguish from the outside from 400 #48 heads, and must be measured. These heads are the 1969 version of the 1968 #18 heads.
Round port heads were VERY different, and were designed to flow a lot more air than the d-port heads, even though they have comparable chamber and valve sizes to D-Ports. They have larger intake ports and very different exhaust ports than D-Port heads. (Though early RA II heads I believe had similar intake ports to the D-Port heads.)
The three primary drivers for detonation are the chamber volume vs. displacement (Static CR), the intake closing event (Dynamic CR) and the quench volume (which is dependent on the bore and the deck height). The article referenced in the hall of fame section boils down to running the dynamic compression at 160 or so PSI. The dynamic compression ratio is influenced by the static ratio, but is more dependent on the ICE. You can run Iron heads at 10:1 on pump gas with the right combination of Dynamic CR and minimum quench volume. TOHCan is the most knowledgeable source I've met for squeezing high compression out of pump gas with tremendous detonation tolerance. It takes a proper set-up on your distributor and carb as well.
If you attempt to run 93 octane pump gas with a basically stock set-up, you should probably limit yourself to about 9.5:1 static compression ratio. Of course, adding the camshaft choices and possibilities to this discussion could change that, but most factory cams have a rather early ICE which tends to raise the dynamic CR. Longer duration cams will drop the Dynamic CR, but could affect the vacuum level. If you are running power brakes, this can lead to some challenges. One solution for that is Rhoads lifters, which shorten the duration of the camshaft at low RPM, boosting idle vacuum.
