An impact is common for this repair, and I've used one... but to avoid hammering the gear teeth I much prefer holding the yoke and using a long handled ratchet on the nut (or torque wrench for installing). It's also near impossible to properly torque the pinion nut with an impact, at least the "torque" is just a guess, even if an educated one.
One way to hold the yoke shown above, other people use a large pipe wrench, but that can damage the yoke.

Try one of these
Pinion yoke holder tool, to remove or torque pinion nut http://www.ratechmfg.com/Yoke%20tools.htm

Also works for drive flange on back of transmission, and fits many other vehicles and applications.

Here's the shop manual for servicing these axles.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0410.htm

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-