you "could" keep the old box but it may not last long term

what most people do is buy a donor 2.9 4x4 and then use the box, props and front diff and sump from that as it's much beefier

the 2.8 front diff isn't interchangable with the 2.9 one, even though it's supposed to, much easier to eliminate the issue by bolting it all onto the lump in one go

don't forget you'll need to use the 2.9 flywheel and flywheel bolts