Hy Iron Clad, if I did my figuring correctly an overdrive transmission with a 0.73 overdrive ratio will give you a top speed of 61 M.P.H @ 3000 R.P.M. with a 7.2 rear axle ratio and 9 X 22.5 tires. Your available power in overdrive goes way down, just be aware of that. A Spicer 3153 is a pretty old transmission, and while I'm not positive, I think you might have to change bellhousings to mount it to that engine. The Dodge version of the NV4500 will bolt to the same pattern bellhousing you have now, but I think it requires a 1" spacer between the trans. and bellhousing. Google NV4500 for more info, there are Chevrolet specific NV4500s but it seems to me the Dodge designed transmission is a better swap for a reason I can't recall at the moment, hope that helps.
