High Compression 390

I would not "overestimate" the effect of the lower air density at higher altitudes here in CO, that lower density's effect on gas engines and detonation is offset by our super low humidity and high summer air temps. I had chronic pinging issues in my old 360 until it finally busted a ring land and cracked a cylinder; it was a 1975 360 LA block bored .060" over with KB flat-tops at zero deck, stock crank and rods, stock Magnum heads and rockers, Lunati Voodoo 256/262 adv. duration hydraulic FT cam (too small), Air-Gap intake, shorty headers... It measured out to 10.42:1 SCR and I had cranking compression only around 160 psi but on hot summer days even with fresh premium 91 in the tank and total mechanical advance of 32* it would ping like a mf'er if I opened up the secondaries in 2nd or 3rd gear (904 trans with 2.94 rear gears). This was when I lived in Denver, then the first year or 2 I lived in Fort Collins so the elevation was usually 5000-6000'.

After that experience I decided I would only go through the effort of a full rebuild for a race car or something really special; having blueprinted high compression and all the associated machine work for the short block just isn't worth it in a vehicle that gets driven all the time with lots of street duty IMO. The satisfaction of having custom-built the engine to my own personal specs wore off after a while especially after seeing so many successful low-compression/junkyard builds on FABO that made more power than my combo. I would have given up 20 HP just to get rid of that damn pinging, the engine would probably still be running today; in hindsight I should have NEVER put that tiny cam in it and just gone with proper duration from the beginning but the high compression still wasn't worth the trouble. Now I'm going with a junkyard 5.9L Magnum and some new top-end parts but no machining to the short block, just because I want to DRIVE the thing. I'd also rather put the money for machining an SBM wedge into the G3 5.7L Hemi swap I will eventually be doing to my Duster.

Since it sounds like this engine will be working pretty hard and get used a lot I would go for lower compression and just use a slightly smaller cam to suit. If it was going in a weekend-warrior type A-body I'd say go for it but 11:1 in a farm truck seems like a gamble to me IMO.