I think you'll find that the fit of the transmission bearing retainer into the hole in the back of the bell housing is important for alignment of the transmission. A proper adapter or spacer will have a matching hole and lip to maintain this alignment. The mounting bolts will allow too much movement and will eventually damage the input shaft or bearing of the transmission.
Somewhere here is a post about using a Jeep T-5 input shaft. It matches the splines of the original clutch disc and is shorter by the right amount to make the trans a bolt-in. Worth looking into?

Current fleet (subject to change w/o notice)
\'49 GMC 3/4-Ton , 60 Austin Healey Frog-eye Sprite (some assembly required), 2011 Dodge Avenger, 2015 Jeep Cherokee. No, they don't all run.
My other ride is a (B737)no, (B767)no, A320.... Update... was Embraer E190, now Boeing B787.

Knowledge is a wonderful thing, but ignorance means you don't know you can't do something.