Welcome back to the fold (here are some tips)
$250.00 for a Savage 21 isn't bad, especially with the Wasp thrown in. I don't know what condition the truck is in, but here are a few tips.
While you're doing the diff mod, pick up the new Savage X bulkheads and shock towers. The new bulkheads allow you to access the diffs much more easily than the old pieces, and the new shock towers are needed to install these. Plus, the towers have three mounting points. You will need two of some of these part numbers to do both ends of the truck, so download the Savage X manual from HPI and the rest will be self explanatory. Part numbers are 85235, 85234, 86048, 86803, and Z649.
Also pick up the new X radio box. It makes life much easier. Part 85236/85237.
Do the throttle linkage mod. For it, you will need an inch long servo arm, the Ofna parts pack (OFN10705), four set screw collars, and one long linkage rod with threads at one end (this will replace the original throttle link).
Bend the rod in the shape of the old rod, with the threaded end pointed to the rear of the truck. Install the servo arm, and mount the throttle swivel midway down the top of the arm. Then, mount the brake swivel all the way out on the end of the bottom of the arm. Secure the throttle link to the swivel with a collar behind the swivel and the new return spring and the collar in front. Repeat this for the brake link, but use a half inch of fuel tube instead of a spring. After this, adjust everything to your liking.
Sorry for the length of this. Let me know you have any questions.
Kappa