Matt,
I did the swap on my 90, from a 302 AOD to a 351W C6. Below is a list of the steps you would need to consider:
1. I first found a donor truck with a 351W and C6...I believe it was an 86. Took out the tranny, linkage, transfer case and drive shaft. This is probably your best bet to get all of the hardware needed. I got the C6 rebuilt for $350, luckily the transfer case was fine.
2. The swap is fairly straight forward...You'll have to take off your C6 linkage, and replace it with the AOD linkage. Also, the C6 and the AOD are different lengths. You can use your existing crossmember and mount, but you'll have to drill some new holes in your frame.
3. Obviously if your tranny is a different length, your drive shaft will be also...A good driveshaft shop can lengthen/shorten for under $30.
4. I'm assuming your 85 is carb'd, but I could be wrong. This is where someone else will have to step in, because I'm not sure if they made AOD's with mechanical kickdown linkages. On my AOD it was electronic, but I swapped to a carb'd motor so this wasn't a problem.
5. The wiring harness for reverse lights and other components will have to be rewired, but shouldn't be too bad once you identify the wires and use the AOD harness.
I'm sure that I missed a bunch of things, so if you have any questions just ask! The only concern I'd have with this kind of swap is the strength issues of the AOD. The C6 is fairly bulletproof behind the 351W, but the same can't be said about the AOD. There are no doubt many people running AOD's with a 351, but most of them have some beefier components inside. If you're serious about this swap I'd consider building up the AOD a little bit more.
-Kyle
