We would like to swap out our six speed manual to a 7. 8, 9 or 10 speed stick trans. We have a 1989 GMC Toterhome (class 5) and weigh 32,000 gross. The truck has an 8.2 Turbo Detroit tiesel with water-methanol injection. The problem is that the 6th down to 5 th ratio spread is too large. Would like to go with a ten speed to reduce the ratio spreads. Our six speed is an Eaton-Fuller model TA (Brazil). How would we find out what other transmissions would fit our existing bell housing-clutch splines and input shaft pilot bearing?