What would be a good price?

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).

In my area of Pa. a shop usually gets anywhere around $600. just for the motor install. That's removing the old motor and installing the new one. That would not include installation of the transmission with clutch components. It would not include set up or any new parts either.

Depending which shop you deal with there would be extra charges for installing intake, carb, exhaust manifolds, and any other parts.

Do it yourself and save a big chunk of change. It's only a few bolts. You will gain some experience doing a motor install in a II.

I hope you don't live in N.J or N.Y. the price could be much higher.