The NP208C from a TH400 or SM465 transmission truck will mate up to the NP24x & NP26x extension housings on the 4L80E used in the T400 & T800 chassis trucks...
The six bolts are not evenly spaced. One is offset to key the clocking position of the transfer case. The LS powered GMT800 rigs are Left Hand Drop the 73-91 CK RV squares are Right Hand Drop so the clocking position will not work even if you are OK with just using four or five of the six bolts. Advance Adapters, and I believe Novak, make clocking rings. One or both probably have LHD to RHD correcting clocking rings or a custom transmission tail housing that will allow you to install the RHD case with all six bolts.
The TH350, TH700R4, & 4L60 have 27 spline output shafts. The SM465, NV4500, and TH400 have 32 spline output shafts. I believe the 4L60E is 32 spline as well unlike the non electronic 4L60. If your NP208C came from one of the 27 spline transmissions you'll need a new input sleeve/gear.
JTR (Jags That Run) should have a VSS adapter that will allow you to keep your cable speedometer and still run the LS motor.
Irish
Didn't some of the 1991 3/4 ton Suburbans come with the NP241 & 4L80E? Scarce as hens teeth...