You're going to need rear fender support. You can reuse the rear brake if you want to, but there are much better systems out there. Of course, all kinds of tiny things for brackets, mounting, etc. Probably need an oil bag, tank mounting, etc. Every rigid I've built, I've put it all together, creating or finding the parts necessary as I go. I basically build the whole bike until I'm happy, then take it all apart and work out the cosmetics (molding, welding, paint, etc.). Then, it all goes together and looks oh so good.
You are going to have to get a builders title since the VIN goes to the frame on 70 or newer models. So when you buy a frame get the MSO with it and get receipts on everything. Once done you can sell the titled stock frame and get about $1200 for it.
You can also just get a weld/bolt on hardtail and keep your same frame and title. You can even rake a stretch your stock frame dont forget and avoid title issues. About all that needs to stay is the neck with the numbers.
Shovel heads where produced from 66 to 84. Only a few where produced for foreign sales and police departments in 85. If you have a bonified 85 you may be messing with a collectors item that would lose value if you modified the frame.
take the rear shocks off and put some struts on it. ride it for awhile to see if you like the rigid ride. if you don't like the ride put the shocks back on. it's a cheep way to figure out if that is what you really want to do.