It fits. I had a friend put one in a 64 Polara [B body}. The motor mounts are easy. a 57-58 392 use the same mount ears as a 318. Small block mounts are a direct bolt on. An aluminum 727, or 833 4 speed bellhousing is a direct bolt on too, but, you need a spacer kit because the 392, like all 61 and earlier Mopar V8's, has a crank flange that sticks out further than a 62 and newer V8. Most of the early Hemi specialist shops sell these adapter kits.
Yes, an early Hemi is heavy, but if you use aluminum parts like intake, water piump etc, plus tube headers, it will be no heavier than a 440.
Width isn't an issue, if the engine is installed from above. The 57-58 392 is wider than a 426, but it does clear.
One of the main reasons the 426 was made narrower than the early Hemi's, was for assembly line installation. It had to be narrow enough to pass up, between the frame rails of a B body.
All Mopars of this era had the body dropped over the engine, so whether it was a 225 six or a 426 Hemi, the engine and trans was mounted to the assembled K frame, and the body dropped over top.
In fact, the 392 Hemi has a superior cylinder head design in several ways, compared to a 426 Hemi. The 392 has a virtually direct flow from intake through exhaust, where the 426 has some bends and bumps, necessitated by the intentional narrowing of the package, for assembly line ease.

As for performance, just look at all the records set in the 50's and 60's by 392's. These engines have incredible performance potential, so depending on budget, you can make it as fast as your wallet allows.