The Iron Division replacement hull is the way to quickly remedy the BIG problem with the Hobby Boss M4A1 kit (see the Reviews section for more comments on Iron Division products).
The other less significant problems with the turret can corrected easily by using an Iron Division all-round vision cupola and a Gasoline or LionMarc barrel (or an Iron Division replacement turret and aftermarket barrel if you want to go the next step and use one of their replacement turrets).
The kit mantlet has a bit of an under-shot look, but some foundry’s seem to have cast them that way (the shape of castings in the Sherman series seemingly varied from foundry to foundry and over time). The commander’s hatch is just oversize though, and the 76mm barrel is anemic. Again see the reviews section for comments.
Iron Division products are available on their site, and both replacement barrels in the products section of this one.
Oh, you may also want to consider a set of Tim Perry's Fighting 48th resin tracks for a little variety (or use them on a Tamiya and use the Tamiya plastic tracks on the HB kit).
No longer a $10 tank but lots of fun
Konrad