Justin,
All non-RA 400 4-bbl engines came with the #16 heads with 72-75 cc chambers and rated at an advertised 10.75 to 1 compression, which, if stock and original still, was closer to 10.0 to 10.25 actual compression, certainly enough to put you in detonation territory. If it has been rebuilt (most likely) to stock specs with clean-up cuts to the block deck and head surface, the compression could be in the low to mid 10's. The cam will have an effect on it's detonation tolerance with a larger intake duration able to bleed off cylinder pressure which will help some. A cam like the 068, or milder/smaller, will keep the cylinder pressure higher and more likely to cause detonation under less load.
The H.O. 400's came with the long branch D-port exhaust manifolds and 068 cam on 4-speeds, 067 with TH400 autos. H.O.'s do add value by at least 10% over non-H.O. 400 Firebirds if they are real documented cars IMHO.
Dennis
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