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

The 7-1/4 is marginal with a slant 6 and inadequate with a 273. Add any additional torque and the chances of a 7-1/4 living are reduced. IF you avoid stressing it with unequal wheel traction in such conditions as rain, snow, sand on pavement, acceleration around corners etc. it might live for a while. If you think you are going to exercise the torque of a 360 you will most likely be able to examine your spider gears from outside the case. Drive it easy and it may last for a while, but plan on an upgrade to, at least, an 8-1/4 sometime in the future.