Miami, if your bigfoot has all double chain throughout you should see something like 19 and 24 tooth sprockets, I guess on a bigfoot it would be 2nd and third axles with the 19 tooth. If you have some single chain you may not use them. I think that's what Robinhood is talking about. If you used a 19 on the countershaft it would be a big gearing difference. However unless you do all flat terrain you might not like it.
If your trans says MU on the top, and you have 25" tires, I'd make sure the clutches are working right(the belt goes all the way to the outside of the engine clutch) and do engine/govenor mods. That should be plenty fast, unless you are on very flat and smooth land all the time.
To Invent, you need a good imagination and a pile of junk. (Thomas Edison)