I have a 67 Mustang Convertible with a 4 speed. I replaced the cam in my 289 and the RPM's are running around 3,000 in 4th gear. My mechanic wants to switch out the 4 speed for a T 5 to get the rpm's around 2,100 when I'm in 4th gear. I have the stock 8 inch rear and wanted to know if i options to get the rpm's down. Can I switch the gears out in my stock rear or install a 9 inch rear. Both are cheaper that installing a new T5.