I think HTC would be better of adding the variant with the M3 Stuart turret instead of adding HEAT rounds for the LVT-4. I'm not convinced the HEAT rounds would be worth it seeing as how the sights are good for nothing but lobbing 75mm HE shells. The 57mm of the M3 Stuart would at least take care of the M18's and M8's with ease, the Panzer IV's and M4's within 800 yards, and otherwise be a PITA for defending ground forces.
I know there were HEAT rounds made for the 75mm, but I have to wonder just how much they were issued.