I am trying to help a little bit here. This design of a 2 1/2 inch downpipe is not worth spending your time and money on because it retains the 2 inch turbo elbow. All the restriction happens at the elbow so a larger downpipe will have no noticeable effect. Attached is a PIC of my spare 2 1/2 inch downpipe which does make a difference.
When I worked at Midas I made a 3 inch down pipe off the bend card they had for the stock down pipe. I welded a flared 2 1/2 end on it and it worked great. I threw it out when I put the hooker 2 1/2 down pipe on it. I should have kept it. Im sure Midas still has bend card for the down pipe. Real easy to make with that card and the bender. They dont even use benders that much anymore .I used it all the time back in the day.
My bad didnt see Hot air section. Midas had cards for those too. A good exhaust guy could make that
RJC does not make a downpipe for Hot Air cars. Only Gbodyparts. RJC used to(now not on there website) sell the flange with the puck which allows you to have a custom downpipe built. That is what I did. RJC is on the vendor list. The 86/87 flange is identical to the 84/85 bolt pattern.
I know it's been 2 years since your post but I'm hunting for a downpipe for my 85 hotair. Wasn't sure if you were still making them or not. I'm hunting for a 2.5 or 3" stainless with the 90 elbow delete (straight) at the turbo...Also a converter delete/test pipe. Let me know if your interested