One of the hardest to beat inline engines for the new OHV V8s of the 50s was the 308 cubic inch Hudson Hornet engine. It was a flathead inline 6 with a bore and stroke that in todays world isn't in favor for a high performance build.
Bore of the 308 is 3.8125" and the stroke is 4.500". I can attest to the unusual bore and stroke as I disassembled one of these engines as a teenager. The connecting rods were extremely long. They measured out at 8.125 inches. I realize that the engine the OP is building is unusual and not run of the mill for a Pontiac build, but I wouldn't discount the specs as impossible to be in a high performance build considering how fast the Hudson engines were.
The other thing the Hudson has in common with a Pontiac is the tall deck, 12.375 inches from crank center to the top of the block. One other dimension is a 2.250 rod throw.
The thing that gave them an edge was the long stroke that produced torque at a lower RPM. This build should do the same thing. I'm looking forward to the end result, hopefully a dyno run to see where the HP and torque end up at.
With a 4.25 stroke his piston speed will only be a small percentage more than a 455, with less reciprocating weight hanging on the rod because of the smaller bore and piston size. Plenty of 4.25 stroke large bore engines have been built, and no one bats an eye when they stroke a 400 block to 4.25.
This should be an interesting build, looking forward to it............
