[CudaPete]

I believe it had a mechanical cam with adjustable valve train to help with higher RPM's. The 7-1/4 sure grip is a pretty stout unit and should stand up fine to a 273. It's actually a rare bird. I would keep it.

 

[CudaPete]

The 7.25 with sure grip is substantially different and more robust than an open 7.25 rear end. It should be able to handle your 273. I have the origininal open 7.25 behind behind my 318 with 300 HP. Haven't broke it in 11 years. They are not as weak as made out to be. All stock 8.75 A-body axles are small bolt pattern. I have one in my garage for when the 7.25 finally gives up the ghost...........

 

[CudaPete]

You also have to re-use the 273 exhaust manifolds for clearance.