That's an impressive accomplishment!
I think its pretty earlier to accuse anyone of "cheating" when no one seems to be exactly sure what the rules are and what he was running at the time. Until someone posts a copy of the rule book showing that only production heads are allowed I think the guy at least deserves the benefit of the doubt.
EDIT: I did about 5 minutes of searching on the internet and this is what I came up with.
The Class designation of BGC was listed as this on the SCTA website:
GAS COUPE and SEDAN - /BGC, /GC
This class encompasses coupes, sedans, and convertibles which must have an engine swap, quickchange rear end, or a non-stock supercharger, any one of which makes the car ineligible for competition in Production class. Gas Coupe class includes Camaros, Barracudas, 1958 or later T-Birds, Mustangs, compacts and other cars of this type. 500 must have been produced yearly.
Engines classes allowed are: AA, A, B, C, D, E, F, G, H, I,J, XF, XO, XXF, XXO
The D engine designation was listed as an engine of 261-305 cubic inches.
I am sure there are more rules but from the class designation listed on the the SCTA webiste it doesn't say 100% production engines only and it even allows for engine swaps.