Randy,
You are thinking in the right direction, Actually I think Mondello used to do something similar with chevys back in the 60,s. can be used to your advantage when you incorporate anti reversion dams in the spacer. Still going to get caught with the height of the collectors in relation to the crossmember over the top of the transaxle and clearance to the rear clip.
Here is another thought.If someone was to build a set of 180° exhausts in a semi finished state that still have to have the primaries welded up in that portion where they run across the car @ 90° to the centerline things could be simplified. These would have enough tube so that for the 'Hi-Port' 351w application would be a virtual weld only job , while 289/302 ( std port height ) would require the most pipe trimming, then you only have to cater for the different primary dia's that no doubt someone will want!
Jac Mac