New here and looking for advice with my setup.

IIRC 255/60-15s are about the biggest tires that will fit in a 67/8/9 wheel tub.
And I can tell you that;
my stock 70 Swinger340/A833/3.55s would usually spin it's G60-14s thru 2 gears, and sometimes into third, in any case; well past the speed limit. G60-14s math to ~248mm , and rounds down to 245. but the Gs had more tread, flat to the road, than the metrics.
Your car with an original 340/4-speed, should have come with 3.55s, at the least.
But to be truthful, those PolyGlass Belted tires were pretty useless. Cold, they were roller skates. Just changing lanes in a hurry could be dicey.