3spd Gearbox pulley & chain what speeds are these good till?

id like to build a racing mower and insted of going through the headache of jackshafts and that for when i lower the rear end i was thinking of going with a 3spd gearbox i have off a old murray rear engine. and then run a solid gocart axle in the rear but my question is what speeds are these gearboxes good till? i will be adding gearoil in it instead of grease.

I saw on heymow some guy used a 3 speed trans that came on a 70 toro - other then getting some upgraded parts for it, it's basically a standard trans.

Here's the link:
http://www.heymow.com/index.php?PHPSESSID=ukdvferp5j37thsmi05f0053k0&topic=14692.0

This is the gearbox i have its a foote 3spd