Things are tight. With the transmission in place I set the seat on top of the tailshaft. Then I took a tape measure to see how much head clearance was left. From the lowest point of the seat to the roof I measured approximately 33". Then I sat in the seat and measured how high my head was from the bottom most part of the seat. 33" again. - And that's without a cage or wearing a helmet.
There are a few areas that can be adjusted. The roof patch is very thick above where the driver sits. At it's lowest point it's probably an inch and a half lower than it will be once everything is back in shape and it's ground down. Another way I think I can gain clearance is by moving the seat further back. The body really has a wedge shape to it and I think I'd gain a few more inches of head clearance if I modify the chassis to allow the seat to slide back another 6-8". Legroom was less than I wanted anyway, so the farther back I adjust things - the more comfortable it'll be.
I'm planning on fabricating all of the cross-members and transmission mounts to be beneath the transmission. I want to be able to remove the transmission from the topside when done. I think I'll also make a stout (bolt to the frame) removable transmission cover to use in conjunction with a transmission blanket. I am concerned about sprag/drum failure causing a big explosion between my legs. The driveshaft will get some beefy loops as well. Weight isn't a concern as much as safety.
I may be able to use measurements off the Dana in the Barracuda to establish the position of the rear end yoke. If that works out I'm hoping to start fabbing some of the roll cage soon.