Don't take it as the gospel, but if I remember right, the input shaft length on the 390's is the same as small blocks and not that of the 428's and 427's. So I would think that if you use the spacer for the small block on the 390. I think you'll be o.k.. Any body can add any info?
You can do anything you want to......ONCE!
aka "my 66 coupe"
All big block S/S transmissions are about 5/8" shorter than the small block parts. A 1" spacer is required on a Tremec to an older small block bell housing, I would imagine a 1 5/8" spacer would be required for a Tremec in a 390