The degree kit you order doesn’t really matter in my opinion, the only difference is that plate that goes under the passenger side perch and I am not a fan of it. I didn’t use it in my conversion and slowed me to drop that spring perch 1/4-1/2” on the axle tube. If you want to use it you have to grind the hell out of it for it to fit.
That steering box will work, and an 80 TRE will work with the 80 box obviously, but it won’t fit into the 60 passenger side TRE. The tapers are different.
$150 is a lot for two TREs.. Like I said, it may just be more straight forwards to convert everything to the GM 1-ton ends with the Ruffstuff kit.