I sort of have a very small little dog in this fight so to speak as I know these bus folks.
It's called Money do you have enough? To let the shop repower your bus with a running take out 6V/92 turbo or rework the 8V/71 and go on down the road with a known warranty and know what has been done to the engine and other components.
Folks Nimco is no longer Nick Liberti has retired from Nimco so we have lost our good source of good used takeout engines and transmission's.
How many times we have seen Quote!
Oh I have a good low time engine that was running fine when it was removed from the bus!
Only to find out it's no better than what we had in our bus before it road failed.
My idea is rework the 8V/71 rod out and repair the radiator install a good Hayden Transmission oil cooler and go down the road from there.
So you go up the hill's a little slower and have less power Than a Silver 6/V92 but you still get there and perhaps have a little money left over for repairs on down the road.
Remember your Kitty Cat knows the best way to solve the problem it's called I think I will take a cat nap and hope for the best.
Good luck on which ever option you go with.