You're probably looking at $50 per hour, so the less work the less money. For example, if your Mach 1 is a 302 with manual trans, the swap should not be a big deal. If you have a 2.8L with auto it will be more.
The actual engine swap will only take a few hours. Its connecting everything up that will burn up the time.
Here are some things to consider:
Ignition-can you reuse the MII DuraSpark distributor?
---You'll need a new drive gear if the '90 is a roller cam engine.
Fuel-does the '90 use an electric fuel pump or engine driven?
---An aftermarket electric fuel pump will be required if the '90 uses an electric pump.
Accessories-Does the '90 use V-Belts or serpentine?
---If serpentine, you'll have to stick with the '90 water pump, PS pump, and alternator.
Cooling-A 302 MII radiator should be fine if in good condition. You'll want to save the factory 302 hoses and reuse the upper radiator neck with the 5.0L.
Mounts-Good condition MII 302 engine and tranny mounts should fit fine with the 5.0L and T-5, but the tranny mount will need to be moved rearward several inches.
Exhaust manifolds-MII 302 will fit best, Fox shorties are out, factory Fox cast iron might fit.
Drive shaft-The MII driveshaft will need to be modifed to fit with the T-5.
What is this engine out of? I thought that the carburated HO 5.0L engines ended production in '84-'85 (other than Ford Racing crate motors).