That is fascinating! I never knew that! I always assumed they had big blocks and small blocks like Chevy. Well, the 400 has excellent torque, which makes a very comfortable ride. My Chevelle has a 406 SBC that screams to 7k RPM. Lots of top end power!
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There you go, now you know the "Pontiac Secret." A Pontiac, unlike a Chevy, is built for torque, just as you have experienced. The best, and most affordable way to really increase torque, it to go bigger cubes - just like a Chevy, except you don't have to source a completely different block family and all the parts to go with it.
The Butler Pontiac 461CI stroker for the 400CI block is the best bang for your buck and the power limiter then becomes the heads. But many aftermarket options if you wanted to go that route. But, like any big inch high HP/TQ engine combo, you will want to then beef up the trans and upgrade out of the factory 10-bolt. Add the upper/lower control arm braces, and hope you don't get stupid wheel hop because then its time for so no-hop bars or other rear end suppliment like ladder bars.
I like a high winding 7K small block, but to get a Pontac to do that and live takes a lot of $$$$. So you build for tire shredding torque at lower RPM's, match your driveline to the power band..........and hang on.
Here is one of our forum member's 400CI/490 stroker build, 10:1 compression, 850 vac secondary carb, repop cast iron Ram Air exhaust manifolds, running on pump gas and going into his 1970 GTO:
Ohio Crank forged 4.50/3.00
KB Pistons 943-30 4.5 stroke
Eagle Rods 6.700
KRE Heads 85 cc stock heads
MSD Dist. 8528
RPM Performer intake
Comp cam 235/241HR14+4
Trend pushrods 8.800
Peek HP was at 616 HP @ 5,400 RPM, peek torque was 703 ft/lb at 3,600.
One thing I do appreciate is your honest comparison of the 406 small block and torque of the Pontiac. The LS Chevy swap gets done on these old Pontiacs, and I get it, but until you have experienced that raw HP/TQ and unique sound that a well built Pontiac provides, I say don't do the LS swap because you just might like the factory parts better.