3/4 Ton rearend choices for 1966 GMC

[runningiron]

I'm thinking about buying a 1966 GMC 3/4 ton that has a rebuilt 250 inline six with a 4 speed granny gear, new clutch, pressure plate. The seller doesn't know the rearend gear ratio, but tells me it doesn't like freeway speeds. I would like to swap out the leaf spring full floater Dana 60 to a later model GM semi floater 14 bolt rearend with highway gears. Anyone have info on what year 14 bolt would fit in the leaf spring 1966 GMC? I'm hoping there might be certain years that are a bolt in. I also plan on installing a Clifford 4 barrel intake and headers and would like to run a 700R4 trans. Thanks, any info would be greatly appreciated

[kieth]

Most common would be 4:10 send me the height of the tires from the ground to the top of the rear tire in inches, then pm me your email and I will send you a spreadsheet that shows how your truck now performs and how it will perform with the 700r4,

attached are some initial performance sheets that ACK has been nice enough to convert to a pdf format for me.......your 3/4 ton probably has 32" to 33" diameter tires so look at that chart, you should be able to see speed at 2000 rpm and how much torque is available with your 250.

https://www.dropbox.com/s/8b4vikws7r...und_master.pdf


I checked and it is going to be marginal with the 250 and the 700r4 we use 460 ft/lbs of torque to the ground when evaluating a powertrain set up. the 6 cylinder actually has more torque available at lower rpm so it probably will be ok with the new 4 barrel and the headers. We just need to make sure about your tire diameter, then check torque again at 2000 rpm.......Kieth

[runningiron]

Thanks Kieth. I haven't bought the truck yet but hope to look at it this weekend. If I do snag it, I will email you with regards to tire height and possibly the ratio. I read somewhere that 3.73 gears behind a 700R4 is a good choice. I'm sure the tire size has to be factored in also. Thanks again.

[Captainfab]

I do not know of any year of 14 bolt either full floater or semi floater that will be an exact bolt in swap. If you get a '73-'87 full or semi floater 14 bolt, there may be enough movement in the springs to bolt it up to the springs. I have done that with a 12 bolt. But you still have the different shock locations to deal with. But with a torch and a welder that is easily fixed. With a '88 and up semi floater 14 bolt the spring pads will also need to be moved.

A Dana 60 is a very good differential, with easily obtained parts. Why not just regear the Dana 60?

[runningiron]

Captainfab,

Thanks for the info. I think that rebuilding the Dana 60 would be best Besides it's a no brainer since the parts are still available. Thanks again.

Runningiron