I've recently installed an inline6-300/ New process 4spd combo in my 66 shortbed F100. my problem is lack of highway gearing, even with my rearend ratio at 305. I have an opportunity to upgrade to a fresh 4spd that was a factory application in a 70 GT Torino with the 351Cleveland option. I'm seeking advise regarding bellhousing techs, adapting, or possibly leaving my 300 out of the transition in favor of a 390 or similar V8. Any wisdom for me guys??