If you don't need any more speed (doesn't sound like it) then just a newer model 4 speed would be good.
Look for an SM465 because PTO's will be more available.
Or the older (but newer than yours) SM420, but PTO's are a little harder to find, used is about your only option.
Investigate how the newer transmission will bolt to your bellhousing. I'm thinking the large hole in the bellhousing might need to be enlarged depending on which transmission you use...
Before making a dump bed check with your insurance company. Many flat out won't cover a dump truck, and most charge a higher price if they do.
Grigg
