Well I think I found the answer even though a larger piston has more piston area 4.03" vs 4.25", area =19.8671"sq vs 20.9517"sq.
A larger cylinder bore has less surface area for a give displacement, 408 LA (4.03" x 4") vs 408 B (4.25" x 3.596") 50.6168"sq vs 47.9886"sq
408 LA. 4.03" x 4.00", (2 x 19.8671) + 50.6168 = 90.351"sq total square inch of surface area.
408 B 4.25" x 3.596", (2 x 20.9517) + 47.9886 = 89.892"sq total square inch of surface area.
Practically the same. So should be no reason the 408 B can't have same cylinder pressure as 408 la they basically have the same total surface just the larger bore has more surface area on the part that actually moves (piston).