Maybe it is the bellhousings fault. The clutch slave mounts differently on the 85-87 bellhousings as it does on the 81-84 bellhousings....Worth a look into.
A guy not too long ago in hardcore put a 6 speed /orion case in his 80. Might was to find out what clutch fork he used.
Cutting it...
I dont think your bellhousing is effecting your problems as of right now. But its should be the 85-87 one with 3F stamped on it. You may have the wrong clutch fork though.
Also, Willing to bet you could make it fit by bending out you floor...With a dead blow hammer. No cutting at all. Jut...
The cast Iron bell should be stamped with a big 3F on it. The clutch fork should be a 85-87...
is the problem that its just hitting your floor? If so it looks like cutting is in order.