I'm sure I'm repeating what others have said, but, 1000 lbs should not be a problem for that engine or TC to move it at 5mph,
IF your gearing is correct. The issue is more likely that you're having to run it right on the edge of converter engagement to go that slow, which is what is going to kill your belt. To do it right, your rpms will have to be ramped up while moving that slow.
This engine can run a 25 ton log splitter... to crawl at 5mph you should be fine. The 420 won't solve your gearing and belt destruction issues... it might just make them worse.
Also, I'll second getting a comet brand belt.
This tool is helpful:
www.bmikarts.com
If you have the same forward/reverse transmission I do, you'll divide your clutch side sprocket by 1.3.... because the transmission gears it down for you, not up. So a 10T sprocket is effectively 7.7 teeth for the calculation. With 22" tires, 7.7T -> 60T comes out to 25mph at 3000rpm (I think you should try for no less than 2000 rpm continual speed). You will need more gearing.
You'll have to find the biggest sprocket you can put on your axle and not have clearance issues, and the smallest that will effectively work on the transmission. 85T (the biggest 41/420 I can find) would put this setup at 14-17mph for 2500-3000 rpm. If you drop to a 8T sprocket in the driver, it goes to 11-14mph for 2500-3000 rpm. I don't like such small sprockets, but maybe it'll work for this. That still doesn't get you where you need to be for 5mph... even 2000 RPM is 9mph. The lowest engagement for most of these TCs is 1600rpm. If you're at a lower RPM, you might not reach the full 1:1 ratio on the TC-Driven pulley, which may also help your gearing. The ratio from the TC starts at 3:1 and works its way to 1:1 (I would guess by 2500 rpm in a standard setup, but I don't know for sure). At 2K you might have a 2:1 ratio out of the TC/Driven pulley, so the 85T sprocket with a 8T on the transmission might work.
So, get that 85T and slap it on, it may work with the TC gear reduction. If that doesn't work, you might have to look at building a jackshaft after the transmission for an additional gear down step.
No question, I'd get the absolute biggest axle sprocket you can find and go from there.
P.S. A warning on that transmission, they are not exactly 5/8 they are metric and 16mm... oh so slightly bigger. 5/8 sprockets will not slide on. I went to a different sprocket on my transmission, and I had to get creative to bore it out enough to fit on the output shaft (emory cloth glued to the perfect size, fuel line, and a drill), and also had to grind it down to fit the width. The key slot had to be filed wider as well. Unless you want to go through that headache, you might want to stick with the sprocket options that came with it.