So what happens if you keep the old /6 rear end after 360 swap?

BOOM? Not necessarily. The 2 8.25's I killed died with a horrible CUH-LANK! and then a whole lot of other horrible noises...
I killed one rocking a $700 Volare Road Runner out of a snowdrift, and grenaded the one in my Aspen R/T beating the snot out of it. (Only after I had an B body 8.75 ready to replace it).

Contrary to popular opinion, you can blow up a Dana 60, but just like a Torqueflite, it's ugly when it lets go.
My 3/4 ton 77 D250 had a 440 in it, and one or more of the spider gears exited through the rear cover at 60 mph on the interstate. I coasted to the shoulder, got out and looked underneath, and saw what looked like the Alien had exited someone's ribcage...a real ugly hole blown outwards with jagged shards of metal surrounding it. Brrrr.
Made a BOOM! sound first, then another sound like someone kicking an several cases of empty Pepsi bottles off the back of a truck....
If you intend to beat on your 7.25, take a cell phone with you and prepare to walk! Hope your friends are home!
Best of luck to you,
alan627b