trany swap / overdriver

I was considering doing a trany swap to accomidate speeds over 65... I have a 1970 F-350 w/ a 390 2wd with a 4 speed ( Manual ) new proccess and im not sure of the exact ratio but, I heard of aftermarket overdrive system but any search proved disasterous and results exceded 2k in which im not willing to spend if i read everything correctly. So the second option is getting a transimision with ethier a overdrive , higher gear ratio and/or more gears can any suject a manual in which would match the 390 and hook up with the current drive train? or tell me an estimate price of a aftermarket overdrive and where to locate one? I considered swaping the rear end however, I still want to have a option of towing without shattering the rear end which was already swapped from 4.11 to i belive 3.75 any help would be greatly appriated and sorry for any gramatical and spelling errors its rather late and im tired. Thank you