Will a 1995 5.0 engine work/fit a 1990 5.0?

yep, they are the same heads same shortblock pretty much, the pistons are the same as the v8 explorers from what I hear, but I had a 95 gt and it is the same engine as a 87-93 or a 94 or a 96-01 exploder

The longblock is basically the same at its core but a lot of the external items need to be changed over to pre-93 parts.

Oil pan is the same, heads will accept the 93's smog hook-ups, balancer is the same, so is the flywheel/flexplate.

The intake, the headers, the timing cover and water pump, the pullies, throttle body, along with their associated items like fuel rails and whatnot.

Basically you can use the engine w/ no problem and whatever looks different, change it.