I have a backyard project that i've been working on for a couple of years (as the budget allows) '73 highboy, i have a set of 6" lift springs from "National spring Co." (front and rear) i plan on running 38" to 40" tires, it's not a mud truck or anything of that sort, i'm not going to beat it, it will basically be a hunting/firewood/work kinda rig, i have a HP D60 from a '79 that i want to swap under the front end, so my question is, even though it's a HP D60, with a 6" lift over stock highboy height will my front drive shaft angle be too severe? will i need to or should i shim the front axle? i'm just trying to avoid eating u-joints if i don't have to, thanks in advance.

 

Thats a good question, and it is an easy one to answer. I run almost the identical set up in two of my trucks, and you will have the greatest driveline angle possible. Since the high pinion axle is so much hogher than the low pinion and you will be usiing a divorced transfer case, you will have a great driveshaft angle.
Even with all of this lift, you will still have to shorten the front shaft but this is about it.
Sounds like a real nice swap and with the exception of the steering, will be a bolt in swap

Just a though, this might be a nice time to perform the u-bolt flip in the front. Its easy, and only requires a little extra work. This really cleans up the bottom side of the axle, and the gains in ground clearance are really worth the effort.

 

Thanks for the reply, did i read you correctly when you said i'd "need to shorten the front shaft" by how much and for what reason? i'm not doubting your expertise i'm just curious.

The u-bolt flip is a good tip, sometimes those things hang dowm 2" or more and snag rocks, stumps, etc.

I still have the primitive push/pull steeering in this truck but it functions pretty good, so i'll just leave that to later, though the 78/79 upgrade is in the works.

 

Moving the pinion yolk up shortens the distance between the it and the t-case yolk. Unless you lift the truck a lot the axle swap will shorten it more than the lift will lengthen it. My truck is at least 6" over stock with the same axle and I didn't shorten the ds. There's only 1" compression travel left on the slip and it doesen't take much to bottom it out.

 

If you're running the same axle and lift as i plan to then i can expect similar results so 75F350's recommendation to shorten the drive shaft seems well advised.

 

My red truck with only slightly more lift required that I shorten the driveshaft about 4 inches.
Two things require the length change, one is the larger diameter of the ring gear. This places the pinion yoke closer to the transfer case and requires less length for the driveshaft. Two is the pinion location, since this is higher, it too locates the yoke closer to the transfer case, and once you combine the two items, the length of the front shaft becomes an issue, especially if the truck has the ability to flex a bit.
Since the front diff, is offset. the spline travel becomes more important. The rear is not quite as affected as the rear is since the center diff travels less during normal articulation.
Not enough travel either way, and you run the risk of popping the front driveshaft apart during a driver side extension, and risk crushing the shaft and damaging the t-case under the opposite condition.

Fordworth has it right on the money, but in my truck, I am confident that I would crush the shaft. Mine sees more than 1 inch of spline travel just hitting the bumps when I enter the freeway down by his house. That I-5 can be a little rough. lol.

 

Thanks for the replies, sounds like shortening the drive shaft about 3-4" is the way to go, i'll know more when i get the springs and axle bolted up.