I've never installed a 5-speed D/R into an first gen before, but I have installed the EA81 4-speed D/R into a first gen. So I'm not sure of the challenges that creates. I've also done an EA82 transmission into and EA81 & a EJ transmission in a EA81 (not easy).
You'll need to swap out the bell housing, flywheel and clutch to the EA82 set up. You can also grind out a notch in the transmission or weld in an extension on the steering linkage to solve the clearance issues, you can ether have a one piece drive line built or use the ea82 drive line and just mount up the center carrier bearing (bolt or weld). Then possibly do a bit of cutting, grinding and welding on the transmission tunnel for the shift linkage and any clearance issues you might have. Then build your transmission mount. From my experience this is what I think you'll need to do. If I were you I'd just put in another 4-speed s/r transmission unless you have the time and money to do this conversion.