The attractions to a 4.0 are:
1) it bolts to the same mounts
2)It uses the same bellhousing pattern, thus theoretically the same trans,
though in practice there are "issues" with most of the transmissions.
3)It uses the same exhaust (2.9 Y-pipes bolt right up to the 4.0 manifolds)
Basically the 4.0 is DESIGNED to fit where a 2.8/2.9 has been.
The 3.8RWD is designed to fit in chassis designed for the 5.0
The 3.8SC engine looks attractive because of it's power and torque,
but the fact that it BARELY fits in the much larger MN12 T'bird chassis'
engine bay and doesn't really do anything you can't get a 5.0
(or more easily a 5.8) to do WITHOUT the complexity and bulk of the SC system.
and either a 5.0 or 5.8 CAN be worked on in the chassis.
a 4.0 swap regardless of year of the donor, provided you stick to '90-94,
is an almost entirely "electrical" exercise.
Yes there are details, like radiator and heater hoses, throttle cable and fuel lines,
but other than those relatively minor issues and the physical problems
with taking one engine out and putting another in (which with a 4.0 for a 2.9 isn't materially different from replacing a 2,9 with a 2.9) the job is making the wiring work.
And that wiring isn't so much of an issue that is related to swapping out
your '88 2.9 and swapping in a 90-94 4.0 as it is replacing the '86-88 wirining
with '90-92 or '93-94 wiring.
That is the problem with the fuel lines, because the '89-up 2.9's use the
later style fuel lines. (entirely related to the change from the dual pump
system used up to '88 and the single pump system used '89-on)
so the question of "what year" is actually more related to
"what donor can you get COMPLETE and RUNNING?"
Though I will say that a '90-92 4.0 will be an incrementally easier swap
than a '93-94 engine would.
'95-up would be a complete horror show.
why you ask? EEC-V.
AD