I think the Swiss K-31's, Swedish Mauser's and Finnish Mosin's are arguably the most accurate vintage milsurps. However, I've seen other bolt action milsurps shoot just as well, like some Enfield's, Springfield's and Mauser's. The Arisaka ought to shoot pretty good too, given bullets that match it's groove diameter, which can be weird. IIRC, those have an odd number of grooves which makes it hard to measure. It's will probably be easier to try some of the different factory loads that are available on the internet. Measure the bullet diameter of whichever shoots the best and use bullets that match that when you reload.
The semi-automatics like the Garand was never renowned for it's accuracy, at least in issue form. It's much easier to get one of the bolt actions to shoot. However, the sights on the Garand are exceptional, and that can make up for a lot of other sins when we are talking about practical accuracy. It shouldn't be too hard to get it to shoot 2-3" 5 shot groups at 100 yards. That's very usable.
M1 Carbines (not a Garand) are infamous for their wash bucket sized groups. I'd consider it a plinker or a 100 yard max gun.
Take good care of those! You have some real keepsakes there! I'd shoot them so you can teach the next caretaker how to do it.
