I am replacing the rear leaf spring and do I need a 4 wheel alignment afterwards?

No, you should be fine. 4 wheel alignment is typically done on all-wheel drive or 4wd vehicles so the tires aren't "fighting" each other. You should be fine just replacing the leaf springs.

I have to disagree re:littlehorn's post. Anytime you change a suspension component, you usually change your camber/caster adjustment of the rear end as a result. I would guarantee the toe-in would need to change since the stance will be higher. Get the $60 alignment to save $400 for new tires. BTW, alignments should be done everytime new tires are bought, too, regardless of what kind of car you drive.

I agree get it aligned!

I have a rule. ANY time a suspension part is replaced or removed/reinstalled, the vehicle gets an alignment. Even on solid rear axles, I STILL set centerline a perpendicular lin, construct 6,8,10 right triangles off the centerline and rear line, and then drop plumb bobs off the rear axle hubs to determine: 1) Is the rear end laterally centered under the chassis? 2) Is the rear end centerline precisely perpendicular to the chassis centerline. Then, I work with all the leaf spring attachments, u-bolts, plates, etc. to try and mil out enough authority to set the rear end precisely. ONLY then, can you truly get the front end properly aligned.... I get it close, and then take it to a shop to dial it in to specs.

alignment, all 4, is critically important to the handling of the 81 Corvette. If it is out even a little, it will be twitchy and scary on 2 lane roads. The original factory spec alignment is NOT a good set up. If it is twitchy steering that is the sign. So, Duntov Vette in Texas sets up Corvettes and has specs for road or track. VBP also has improved specs. I used: Rear: (do rear first) .5deg neg each side, rear toe 1/8 total so 1/16 each side, pivot bolt 50ft lb. Front caster right 3.0 to 3.25, left 2.7 to 3.0: Camber .29 neg both sides. front toe 1/8 total so 1/16 each side. (.08d toe in). Nico