I believe an '86 close enough to the changeover probably has both the G50 mounts and the 915 mounts on the body. Are you trying to use the factory G50 cross member?
PMS makes a cross member to mate 915 body mounts to the G50 (eg for earlier cars). It's slotted, which gives you more leeway when fitting. Also accepts aftermarket 915-style transmission mount pillows like the WEVO ones, which some view as a win over the bonded-in G50 mounts/no firmer options.
My 930/60 was already a tight squeeze into the '77 with a 915, pulley very close to the rear body cross-member. With a SBH G50, it would not go in.
Relieving some of the rosette welded plate over the TB casting buys you 10, 15mm or so for the nose of the G50 to go into. Don't know how much you can take off the G50 nose casting, but taking too much won't make you happy
https://forums.pelicanparts.com/porsche-911-technical-forum/440191-verify-g50-shortened.html