370
I want the smaller main journals and the shorter stroke.
A 389 is a 400 less 0.058" bore. When I was building engines it was always, "When in doubt, bore it out." The 370 can be bored to 4.121 and use a stock 400 piston. Use a 428 piston and lengthen the connecting rod. The bore/stroke ratio and the rod/stroke ratio get real attractive. Compression ratios from 7 to 1 all the way to 12 to one are attainable.
The cost for the adapter and motor mounts are the only issue with the 370 being one of the best Pontiac motors.
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