IS there an sort of adaptor or do I need new shafts?

Not an easy job due to the different size drive shafts.
The pushin shafts will not fit a bolt on diff and bolt on shafts will not fit drum brake hubs.
On one of the Sierra forums they discribe how to build shafts with push fit at the drums and bolt on at the diff but you need to find the thicker diesel push in shafts so you can take of the tripod joint at the diff end and replace with the bolt on one.
I have yet to find the diesel push fit shafts.

So my best bet is to get the whole rear assembly?

Do the driveshafts need cutting to length?

I haven't built the car, I bought it road registered and am putting in a different engine, but I need a different ratio, not offered by the 7" diffs at a price I can afford