Swapping rear end question
Where can I get the u bolts and shock plates?
You can get U bolts new online. I know ESPO has them, but I've seen them other places too. You'll need ones that fit the 3" diameter axle housing on the 8 1/4. The shock plates I think you would have to get from a donor car or ask around on this site. Those are also different for the 3" axle housing (the holes are farther apart to fit the larger U bolts).
What's a fair price to shorten the driveshaft? Is that something best to leave in the car and just trailer it to a drivetrain shop?
I don't know about pricing, but it's pretty easy to ballpark how much shorter the driveshaft needs to be, and the measurement doesn't need to be that precise. There's some slop built in because of the fore/aft movement of the driveshaft during driving over uneven roads. (Look at the front yoke on your existing driveshaft and you'll see where the shiny part of the yoke has room to slide forward into the transmission. It doesn't matter exactly how much shiny part is visible as long as the yoke doesn't go too far in/out at full up/down displacement of the differential. If your measurement is off by a quarter inch it'll be fine.)
On the old diff and the new one, measure the distance from the axle housing
centerline (remember the axle housings are different diameters) to the part of the pinion yoke that contacts the u-joint. The difference in that measurement between the 8 1/4 and the 7 1/4 is how much shorter your driveshaft needs to be. In my case it was about 1.5". You can use that measurement to either find a driveshaft the right length or have yours shortened.
It's super easy to pull the driveshaft. When you remove the 4 little bolts holding the rear u-joint onto the pinion yoke, the driveshaft will just slide rearward out of the transmission and fall out if you don't catch it. If you want to leave it in, you'll need to wire or tie it up to the car body, making sure it can't slide rearward, and you'll need to secure the u-joint bearing caps with tape or a rubber glove or something if you plan to reuse the u-joint.