looking to change my axle ratio

I wouldn't go past 3.55 if I were you. Those old engines like to spin a little more than the modern ones. 1500 at 65 would be as low as Id go. My old t-6 ran the same rpm as yours and got 7 to 7.5 mpg doing it. It had an 8LL with 4.11's and short rubber. On those rears check the spindles for wear also before spending too much. A lot of old ones I've seen have worn spindles. If they're really bad you might be looking at new housings.

 

Most of the older eng's were governed 2100 with a low about 1700 below that was considered lugging, its probably a 70mph trk at 2100.

 

I'm not an expert, but when I was looking at re-gearing my truck, my local shop said that you can run the same carrier and switch between 3.55 - 4.10's. I believe to run a 3.42 you would need a carrier from anything with 3.08 - 3.42's.

 

how good is the truck....I would sell the truck and buy one with the correct rears. Swaping them is gonna be a few grand anyway. WHat speed do you want to cruse at? Whats your proorety? High top speed? Mpg? Check out the eaton rear end calculator. http://www.eaton.com/ecm/groups/public/@pub/@eaton/@roadranger/documents/content/ct_062746.swf If mpg is your concern get a rear that would allow you to switch to 22.5 tires.

 

For a shop, I've had really good luck with the guys at Heavy Duty Parts in Scranton. Heavy Duty is a chain that specializes in drivelines, but don't go to any of them, some really suck. I trust the guys in Scranton with our transmissions and rears, I've been using them for years with no issues and they know their stuff. In Scranton, Gene is a straight forward honest guy.