I just bought a 1958 Ranbler American with a blown flat head 6. Will a Chevy 250 Straight 6 fit in?

Best to switch the engine abd transmission. It will require lots of work, relocating the motor and transmission mounts along with other things.

I know this is a year old, but... The 58-63 Rambler American is a reintroduced 50-55 Nash Rambler, and shares the same engine bay size and flat-head six. That little six is very short and narrow. The little Nash Rambler was literally built around that engine, so few engines easily fit. The Chevy sixes and later AMC sixes are just too long and wide (width of the intake manifold, not engine block). The only other I-6 that easily fits is the Ford 140/170/200/250 family. They have the intake made onto the head so they are narrow, and have a small bore with long stroke, making them about the same length as the old Rambler motor. You can use those but must change the transmission with them, and custom make motor mounts and have the driveshaft altered with the Ford front/Rambler rear. The Rambler 195.6 (also called a 196, but don't confuse it with the later 199) OHV six will bolt right in and even bolt to your existing transmission. They are hard to come by in good condition, as the heads are prone to crack if the head bolts aren't torqued every 10-12K miles/three years (whichever comes first). The large head expands and contracts a bit and will eventually work the head bolts loose, causing a blown head gasket and overheating. That eventually leads to a cracked head. Older engine just require more maintenance.