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I've been building a ten4 very slowly on and off for the past couple years. Right now I'm having a problem where the front and rear drivetrain are very free (they will free spin for a few seconds without the belt installed)when separate. But when I install the belt it has a lot of resistance. I am running a short wheel base car and I have a 573 and 570 belt. I have the short wheel base tensioner.

Definitely run the 573 belt. We didn’t have custom belts made, we just used off the shelf parts, so the belts will take some break in to loosen up a bit. The belts have to be run tighter than you would think due to the small diameter of the pulleys. You will get skipping if you don’t. You could remove the eccentric tensioner and run a smaller bearing directly on the screw such as I did on the red car in the pic. I want to say it was a pair of 3x6 or 3x7 bearings. It’s a tad bit less tension than the lightest position on the eccentric which I needed since the strengthening kit tightened the belt up a bit more than stock.

I'm running different pulley's of the same tooth count though. But Mr rear tensioner is different with the 573 belt the tension is very loose. Here is my car. The rear tensioner is just 3x5 flanged bears on one screw of the gearbox and the front tesioner is very dirty because shapeways. The gearbox is printed by me because I cracked the shapeways one tapping it, but it is identical to the original one. The car has the 570 belt on it in these pictures

I have some experience with CATs and Mids and you'll probably never achieve a completely free spinning drive train using a belt. Among other factors, there is significant friction from each tooth as it rolls in and out of the pulley and that friction exists independent of the belt's static tension. A chain typically creates less friction but there are other issues with a chain. For an off-road buggy, I still believe a belt is the best option. The good news is, with today's motors, batteries and ESCs, you can always increase power to offset the belt's friction losses.

So it sounds like I just need to run it the way it is.

I'd recommend a brief test run on the bench so you can confirm that nothing is bound up which could lead to overheating. This will give you an opportunity to check the belt's tracking. The run should also warm the belt making the drive system seem a little less tight. Anyway, that's the procedure I use whenever I've had to rebuild a CAT.