I don't know much about armor, especially the Pacific Theatre. However, it seems to me that Japanese tanks were inferior to Allied tanks both in quantity and quality. Their default medium tank was not much better than the US M-2 medium tank, and was produced at the rate of 300 to 500 tanks per year. A tank like the M-3 would seem to be more than adequate in the Pacific, with a 75mm used like artillery with mostly HE ammunition, and at the ranges in the Pacific the 37mm used as anti-tank would be adequate. How about something along of an updated M-3, or an updated Churchill like you envision but keeping the howitzer like the earlier versions.
Japanese armour quality was, as you say, inferior throughout the war and something like the American or British 75mm guns was more than sufficient. However the 3" howitzer fitted to British CS (Close Support) versions was, essentially, a breech loaded 3" mortar with a muzzle velocity of only 600 ft/s, too low to even pop most Japanese armour, and there was no AT ammo for it, ever, for that very reason.
I chose the 25 pdr for my "what if" mostly because it had an outstanding HE round, much better than the 75, and quite adequate AP performance with solid shot and the supercharge powder increment. It was also being manufactured in Australia during the war, unlike either of the 75mm guns. I then added 10% to the length of the tube to increase the muzzle velocity a wee bit without overstressing the breech to bump up the AP performance a bit more. I also posit that, for tank service these rounds would have to be fixed as opposed to the semi-fixed nature of the normal 25 pdr rounds, but that's not a huge deal at all. The AP rounds would be the same a s the super-charge rounds and the HE rounds would have something like a charge 3 amount of propellant for adequate, but not long range HE firing, they're not artillery, after all. A smart design team would ensure that the actual cartridge and shell of the semi-fixed rounds could be used in the new tank gun chamber in a pinch to retain the ability to use artillery ammo if available or when indirect firing was desired as was sometimes done with tanks during the war.
As you can see, I've given this far too much thought...