Your right on the lack of weight change when turning the unit around, my engine was spaced 5" and yours isn't shows what I know.
I have looked high and low for a different rearend for the th425 but that pinion is awful short and of course special splines on it. A GMC motor home guy went to India and had 3.70 and 4.11 gears made so I bought the 3.70 and while I was at it make a 31 spline trutrak fit. That said I was still looking for a rumored aluminum third member that Mickey Thompson had made for the th425 and that is what I was searching when I found your thread.
Now to answer your questions, built the car in the 80'S It was in hot rod in 1986 sat from 1990 to 2009. Started autocrossing and thats what initiated all the additional mods.
At the drags with 245 tires and 3.08 rear end it ran low twelves our drag strip closed and I always had better traction on the street but after the recent mods and 3.70 according to a gtech device it runs low 11's will take it to sears point sometime and see.
I build a vega it was in hot rod also in the 70's it also had a th425 with the engine on the trans and it handled I thought fine. the fiesta had a 5" spacer to put the axle under the next main cap and it handled MAYBE better. I recently ran the numbers and moving a 450 lb engine 5" on a 90" wheelbase car only moves 25 lbs from the rear wheel to the front. Thats why I am moving the engine an additional 12"
prior to starting this latest mod steady state cornering it would pull 1.25 G on hoosiers and 1.38 g peaks
Correct suspension geometry go's a long way in taming the rear heavy beast.
mileage? no Idea