Want to buy 7 1/4 sure grip

Guido; by the time you add up converting your open7.25/3.23s to, 3.55s/SureGrip, money-wise, you would be well on your way to purchasing an 8.75, which has drop out centers.
Furthermore; from 3.55s to 3.23s is only a 10% change.
If you are looking for more off the line oomph, a much better idea is a higher stall TC (assuming she's an auto).

In your car, if a streeter, the 3.23s should give you a nice Second gear, hitting 60 at 4400 with 24.5" tires and 15% slip................... which would be peak power with a 2-bbl engine. But with 3.55s it would be 4840.
I suggest a 2800TC.
Agreed, except for the 8 3/4 rear. Nothing wrong with a 8 3/4 except price, there are other options, at a lower price point.