The oil pan should'nt be a problem, in fact the Riv pan may be an advantage for you depending on where your front axles are located in relation to the engine. If an 85 engine it should be the 20 bolt pan which is same as 85-7 GN motors so you have options. One other difference the Riv motor has is the motor mount, same bolt holes in the block so you can switch to GN/Regal mounts if needed.
Question on your trans/ transfer case, giving me a possible idea here, how long is the overall trans/transfer case assembly, measuring from bellhousing/ block surface to end of transfer case, and how about end of transfer case towards the front output shaft/yoke?
Could be interested in some parts here, I know of another board member who may also. If car is 85, the chance of the differential being a 3.36 is very remote but if it is I would be interested, the code on the decklid parts decal would be something like FW4 or if looking at the differential there is a tab when looking at the diff cover that will have a few numbers and letter stamped, should have an 'N' for 3.36 gear, 'M' would be 3.15 anything else would'nt matter.Would be interested in a rust free driver fender, dent free ( a ding or 2 ok) driver side rocker molding, motor mount bracket if you switch to GN, TURBOCHARGED 6 emblems if you are'nt going to use, Ttype and Riviera emblems if nice, probably some other stuff if I remember later, and I need to know what state you're in as I'll have to figure if any heavy stuff is worth shipping.
'Turbo Riviera' (Scott N.) did you keep the 84 Riv, just wondering?