Yes, you can pre-assemble the blackice as well. The primary benefit I saw was I was able to line up my Mace optimally with the IM (you can still see and feel inside your spacer when assembling off the engine), so as not to have any rough edges or restriction to flow. There is still alot of latitude in the bolt holes allowing your spacer to move around- maybe not to the best spot when you're doing it "blind" and you can remove any excess gasket maker. Once the gasket maker cures, she's not gonna move.
This may be more pertinent to the Mace, since it's openings are the same size as the IM's.
I don't know the nut size- I'm a packrat and have a drawer full of stray nuts and bolts that come in handy pretty often. If you don't, take an IM bolt to Home Depot and size it and buy a few nuts and washers (to distribute the pressure evenly) to fit. 4 or 5 will do fine.
Don't worry about tightening the bolts down right away- let the gasket set up awhile finger tight ( an hour), then just tighten down a couple more turns til its cured.
You don't need to remove the ported TB, in the morning remove the bolts, and just put the IM/spacer/TB on top of the OEM gasket in one piece. Have that vertical back bolt already in place, partially inserted in the IM to ease its placement.
Then re-insert the rest of the bolts, and tighten to 18lb/ft
.
PS- the back bolts are 13mm.