The 4.266 J1 will be SLIGHTLY slower through the gears than the 4.400 S1.
Also, the J1 will give you a slightly lower RPM for a set speed than the S1 in any gear.
Do not think that this 4.266 final drive will be slow like the much slower LS trannies. The longer gears combined with the 4.266 FD is what makes the LS trannies slow.

I would have to see this tranny before offering you a price.