"If I was to keep the original [enclosed drive-shaft three speed] transmission what changes are needed to run it with open drive?"
You will not be able to use your original (three speed) transmission with an open drive-shaft (oh, I guess you could, with a driveshaft adapter from specific 3/4 ton GM trucks)?
You can use the 3.55 R&P with the original enclosed drive-shaft 3-speed. First gear will be (might be) a little "high" for start-up on a steep hill. I will most likely go back to an enclosed drive-shaft in my 1954 Suburban that now has an enclosed drive-shaft SM420 (to regain the middle passenger front seat).
