273 Stroker, 4.00" Crank
To get decent pistons you need custom anyways for a 273 but most I'd go is the 360 3.58" stroke and still have great rod ratio and decent bore to stroke ratios. Out of the 4 small blocks the bores size is the only real differences and limit for power on each. What the limit is for a 273 who knows but probably ain't that many 350 plus hp 273 running around. I sure someone with the talent and equipment probably could squeeze a lot more if they were so inclined to do so.