Planning a rebuild of an 8 1/4 axle and need a tool to hold the companion flange while tightening the pinion nut. My buddy that had a home Hand made tool moved away. His was a flat plate of 3/8 steel with the flange shape cut in it and a long piece of steel heavy round stock welded on to it. Any suggestions where to borrow or buy one ?

From one of our own here........
http://arengineering.com/products/rear-end-yoke-holder/

Either make one or I would try to use a 48" pipe wrench to hold the pinion flange.
I have rebuild many rear ends and never used a tool like that. I used what was handy to hold the pinion when tightening the pinion nut. Most times I used a impact and holding the pinion was not a big deal.

Pinion yoke holder and puller $28.99

https://www.summitracing.com/parts/rat-18002

I forgot I own this to torque pinion nuts on dragster rear ends. The tool is on the left.