There are also 19.5" wheels that fit your bolt pattern. They are easy to find, look on the 90's Chevy 3500HD and other similar chassis like a P30. They are tubeless one piece wheels with a largest tire size of 8R19.5 that is about 33" tall. To get a tire comparable to 8.25X20 you need the hard to find 22.5" wheels from a 56 1.5 ton Chevy. Then you can run 9R22.5 tires at about 38" tall.
There are no rebuild kits that allow you to turn a SM420 in to an OD transmission.
If you want to go fast you will need more power.
When you decide what engine you want then you can consider transmissions that are an easy fit and offer OD.
If you don't have or want an OD transmission you will have to swap your rear axle for something with fast gears, somewhere between 3.54 or 4.56, depending on tire size, engine RPM, and desired speed.
There are also several choices of auxiliary transmissions to get you OD, such as Saturn, Gear Vendors, and the old Spicer 5831 and similar.
Grigg
