The 8.25" pinion length is 1.6" longer than the 7.25" according to this link (meaning the driveshaft would be 1.6" shorter), so the Explorer rear axle ends up somewhere in between those two. If it's less than an inch off, you might not have to change the driveshaft. Your slip joint might take up that much difference. I'm not sure, though.

I did the same thing Rick mentioned when I swapped a Jeep 8.25" into my Dart. Put the axle in place sitting on the new mounts, and raise/lower the pinion end with a floor jack until your angle is right. Put your jackstands under the leaf springs so the suspension is loaded when you're measuring the angles.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Thanks Frank!

Now, how do I measure pinion angle? Do I hook up the driveshaft and get a compass?

Hmmmm. Here is another question. If I cut and reweld the driver's side tube on the Explorer axle to shorten it, will the axle still be strong enough to jack the back of the car up by putting a jack under the center pumpkin? Any welding recommendations for a good electrode to use and the amp setting on a stick welder to toweld the axle?

This is where an older and wiser guy should jump in, because I'm not sure I necessarily did this right.

What you need to do is make sure the pinion shaft is parallel to the transmission output shaft, so that the angle across each U-joint is equal and opposite. As long as that's the case, you shouldn't have any added vibrations. The driveshaft angle doesn't really matter. (In this application at least. I assume for a custom build you'd be concerned about staying within some maximum U-joint angle, but you won't have that problem here.)

So the important question is how/where you measure each angle. I bought a magnetic angle gauge very similar to this one that measures angles from the vertical or horizontal. For the pinion angle, I put the magnetic base across the end of the yoke where the U-joint attaches. For the transmission output shaft, I used the face of the flange where my driveshaft bolts up, but mine is a '64 that originally came with a B&T joint, so your transmission might not have that. If it has a U-joint yoke directly on the shaft, you could do it the same way as the pinion angle. The only question here is whether the faces of the yokes are actually perfectly straight...

Once you have these two angles, adjust the height of the pinion with your floor jack until the two numbers are the same, or within a degree or so. I had the axle sitting in place on the new mounts, with the U-bolts installed loosely to the shock plates just to make sure everything was in the right position. If your engine is a huge torque monster, you'll want your pinion angle a bit lower to compensate for axle wind-up. Otherwise you might get vibration under acceleration.

Also, this might be a good time to replace your transmission mount. If it's old and shredded like mine was, it might affect your transmission angle.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Just asking, but why won't a 3.71 work? There is not a whole lot of difference from a 3.55. I thought the whole idea of this ordeal was to slow him down?

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Official Cookie and Mater Tormentor.

Right. Slow him down and keep the highway RPMs down. I was shooting for 3.2 gears but got the 3.5 gears by mistake when I bought them at Wildcat Auto Wrecking back in 2010. The 3.7 gears would make the engine spin too fast at highway speeds with a 904. Plus, I know my brouthr woudl get in trouble jetting around Seattle with 3.73 rear axle gears. Burnouts, jackrabbit starts, speeding tickets, etc...

I am hoping to get 3.2 gears. I would accept 3.5, but that is my upper limit (unless I just can't find any 3.2s or 3.5 axles).

Thanks Frank! That makes sense.

Well hot diggity! My brother managed to find an 8 1/4 with a 3.21 gear ratio in a junkyard in Oregon. It will be a LOOOOONG drive to get it, but the price is less than i would have had to spend on the Explorer conversion and I can just drop it on the leaf springs and be done with it. No cutting and welding. Hooray! I paid for it this afternoon and will be picking it up in the next few week as time permits.

So, I guess I won't be doing the Explorer conversion after all, but it IS a good idea if you are looking for an axle to swap in.