the issue with the 4 cylinder one is that it has a tendency to twist...even if welded in...
its just not built to handle v8 torque.
even my stock 4cylinder one was bent when I pulled out the motor and trans to start my swap
its a good temporary one...but you need to go with the v8 mustang one and something more substancial to handle the v8 torque(that means something stronger than the stock double hump crossmember as well)
I always suggest making something yourself or buying a tubular one made to handle some power
I have a TCI 6x, which uses a 4l80 case...so I had to fab up something to fit my car... its quite a bit longer than any of the other transmissions...and it would be a challenge to use any of the stock brackets even if I tried to move them back...
my simple solution is to weld in some thru the floor subframe connectors, and weld on a 1/4" steel plate to each subframe with a small gusset on each one to connect it better to the frame(more than one surface welded)
and I fabbed up a custom trans crossmember out of some square stock.
I actually re-purposed the crossmember I was using on my 2001 Camaro with this transmission... I cut it down and welded back in pieces that made sense for the mustang.