My fabricator (Dad) made up an adaptor plate out of 20mm aluminium billet.
Drilled one large hole the same size as the throttle and 4 holes for the studs and bolts.
He doesn't have MIG/TIG, so we got a local shop to weld it on for a few bucks (shop around, one place quoted $80 for what was a 10 minute job). But i don't know what rods/wire they used.
We just bolted a plate onto the old opening at the back with a gasket, I suppose i can unbolt it if i needed to clean it or something later. Also, there isn't a huge amount of clearance between the plenum and clutch master, so don't make the plate too thick.
Also think about putting on a new clutch master BEFORE you bolt the plenum on, you will need to remove the whole plenum to change it later - on the AE86, AE71 would be similar/the same.
Don't forget to have a look at your throttle cable setup, we made another small aluminium plate to weld to the side of the plenum, and bolted the throttle cable/return spring bracket to that. If you go that way, you can get it all welded at the same time. The other way is to drill and tap the cable bracket directly to the plenum, but you risk drilling straight through the wall and having vacuum leaks.
I've got some photos at home, bug me for them tomorrow if you would like to see them.
Good luck with the conversion dude.
