Take that 400 lock and have it sonic tested, if it has no less than .160 cylinder wall thickness now on both the major and minor thrust surface use it to make a 505 to 512 C.I. pump gas stroker
My first 505 C.I. 400 stroker motor made 612 HP at 5500 RPM and 544 Ft lbs. torque at 4500 RPM with a set of midly ported 906 iron heads with 2.14 intake and 1.81 exhaust valves. they barely flowed 266 CFM at .600 lift I used a low deck six pak with a set of 1970 OEM 440 carbs. and a big Comp Cams soldi roller that was 260 @.050 on the intake and 266 @ .050 on the exhaust with 1.6 rocker ratio.
On your deal the same short block with a better set of new aluminum heads and a good single plane intake would make 650 HP easy with the same compression ratio on pump gas