You my friend are in "Trial by Error" land.
You'll need to pick a gear ratio and try it out, if it isn't enough or is too much you simply buy more gears and do it again. I know this isn't what you want to hear, but is realistically the only way. There is a lot of that going on in 4x4 circles, my cousin uses a Quickchange rear axle in his mudracer just for that reason. If the track is tacky and he can't get wheel speed he just changes to a higher ratio in the rear. He runs different diameter tires on both ends(like 6" smaller on the front) and different gear ratios on both ends, and then just slightly adjusts the rear with the Quickchange to fine tune it. The smaller front tires are geared to be always spinning a lot faster, to keep them cleaned out and pulling the front straight ahead, and the rears are then geared to closely match the pit "suction" that day with the QC gears.
Reason you can't just easily figure this out mathematically is in your case you have to have the tires spinning, not hooked like they would be on pavement, and since you are in a gooey sticky mud pit that weighs things down and are spinning big heavy tires(by racing standards) it is often HP vs RPM vs Gearing vs Tire Speed to get the wheel speed at a point you can maintain it and not get bogged down.
In this case I would say it is better to have too much gear than not enough, I would think a 4.88 front/5.13 rear might be a good place to start.