Actually, IIRC the Russians specifically asked for only Diesel powered Shermans, as they didn't tend to burn as easily. Most Russians positively hated the Sherman. Before the wet ammo storage came along, they brewed up easily.
As for the 76mm never completely phasing out the 75mm, one reason is that HE ammo for the 76 didn't exist in the early stages-76's would have to be escorted by 75's in case of infantry.