Does it Fit!
What ever happens you have a set of rear discs and a 3.90:1 ratio gear set ! Check to see if it has a Limited Slip unit too. Strangely enough mainly the Auto gearbox Impulses have the LSD unit - the Auto was a $1,500 option and they added the LSD to the diff as well with that option.
My GT has been at the body shop for 14 months but is coming home on Thursday so I will be doing this swap too. I have been down at the shop doing all sorts of measuring and:
1) The Impulse sway bar looks a pretty good fit. On my 1972 GT there are four (two on each side) threaded holes in the floor pan each side of the torque tube in, as near as I can measure, the right place to bolt the Impulse sway bar to the GT. I haven't done it yet but it is close.
2)See above - Also you need to use the GT torque tube and inner driveshaft as the Impulse one is a bit longer with a slightly different mounting - some bits may be usable??
3)The Impulse diff is about 3/4 inch wider each side (20mm). It may be usable like it is BUT for liability reasons the spring cups should be lined up correctly. It might even be better to use the GT spring cups and certainly the GT springs as the Impulse is a heavier car - check the spring rates.
4) The GT and Impulse shocks are directly interchangable - I have Impulse shocks on my GT already. However the Panhard rod length may need changing to locate the diff correctly.
Hope this helps - updates as my effort goes ahead