Hmmm. I think you could take that one step further. Instead of just ripping out the gun and welding the port over, why not cut out the sponson assembly entirely, and weld a four-plate section into the gap, creating a flush front? That would significantly increase the internal space. Combine that with moving the radio out of the hull (either into the turret due to it only possessing a two-man crew now, or an armored box welded to the turret rear) and you could likely fit an eight to nine man squad.
The resulting vehicle might look something like this:
(Yeah, I know it's a pretty ugly looking manipulation. It's just to get a rough idea of what it would look like.)
The problem with that is, you'd have to divert pack-howitzer production, and increase (modified) turret production for the M8. Either that or start taking apart Scotts. If you stick with the M3's original turret and 37mm gun, you don't have to worry about interfering with existing demand for M8s, and you're using equipment that's deemed obsolete (assuming that modifications into APCs start sometime in 1943). By the tail end of the war, the M8s were getting replaced by heavier self-propelled guns, and you saw their turrets getting mounted on things like the LVT(A)-4. But that was only later on, starting in '44.
On top of that, you're going to run into ammo storage issues rapidly with the 75mm howitzer - the shells are so much larger than the 37mm rounds that if you want the vehicle to carry a decent amount of them, you'd need to start storing them in the lower hull, eating into space for the infantry dismounts.