..You have a very nice friend indeed. Yes it will all fit on the 73 block with attention to some minor details. I rettrofitted my 73,six years ago but kept the 73 heads for no reason but to save some bucks back then.
..Here are most of the details and plan to spend some money on this conversion.
..Your block is the higher deck so the compression with the late heads will be down due to them having 64 cc chambers and your block having .035 piston deck clearence and dished pistons. To improve this situation,replace the pistons with flat tops and deck the block to match or get a 5L short block.
..Move the battery mounting to the right side of the trunk area using a mtg. kit with wire etc, no room for it up front due to the air intake and filter for the efi needs.
..Fuel--Tank needs to have a sump kit installed or a fuel starve will occurr when accelerating on a fuel level of 1/4 or less, it will uncover the pickup. Fab a mounting bracket and mount a 85/86 pickup truck external pump just in front of the tank. Obtain 3/8 tubing and plumb 2 lines up the right side underbody to the right front fenderwell and thru the engine bay panel just back of the alternator to hook onto the fuel rails at that point. Note that all fuel hoses MUST be of high pressure fuel type and advise using AN type hose and fittings thru the engine panel to fuel rails as there needs to be flex here for engine movement. Hose clamps need to be the wide solid type not the normal general use type. Feed line pressures between the pump and the "in" side of the fuel rail are in access of 100 lbs. Fuel pump relay can mount in the trunk forward the battery.
..Support-- fire wall to spring towers. Must fab new from square or round stock with mounting ends. The upper plenmum will interfer and no power brake vac. booster room.
..Ignition--must prepare the efi housing with 302 shaft if using the 73 block along with the 73 type gear. Check oil pump drive shaft length. 302 and 5L blocks are different in this area.
..Injectors--5L fire order is same as 351 so the injector wiring to the computer must match the 302 order as well as plug wires on the cap.
..ECM--computer can be mounted on a simple holding bracket under the right side of the dash just up enough to be out of the way and accessable.
..Oxygen sensors--should be mounted within about 15" of the ports in the head pipes. Mtg. fitting is 18mm x 1.5 thred size.
..Exhaust--can be stock or prefer an improvment with shortie headers and attendant system. Not reccomend long headers. See the MCCI news letter.
..Cooling-- 3 core air cond. unit with a shroud,and air cond. fan or late model fan/thermo clutch assembly. Bronco II/Ranger will fit if matching holes are drilled in the clutch hub.
..Alternator--90 amp unit with external or internal regulator. This is needed. The efi system now runs computer,injectors,fuel pump,ignition, along with lighting and other features of the car. During normal running about 15amps +/- is being used. There must be reserve power or!@#$% happens.
..Trans.--For auto, the stock TV cable can be made into a kickdown if a double lever is fabbed for the trans end and a bracket to mount the cable at bell housing.
..Neutral safty switch wire into system so engine cannot be started in gear.
..Heater hookup off the back of the lines near the fuel regulator.
..There are other tech aspects of this conversion such as camshaft replacement upgrade (non roller),head work, fan belt change over, that could be considered if needed or desired on the early engine.
..Keep in mind that you donnot have factory backup with this kind of project. You become the expert and trouble shooter when something goes wrong so study EFI and engineer the job well, make notes on how each part it was done at the time. The local garage won't work on it!! There are EFI drawings and a book on the subject. Its not as simple as it may appear. There are perhaps by now some half dozen Mav/Comet conversions that have been done by others that I know of and still others yet out there.

..Iv'e covered most points in this kind of conversion to give an idea what's involved. Painless wiring co. may now be able to offer a kit to help simplify the hookup. Print this info for reference and good luck to all. It can be rewarding with performance along with a WOW factor.
Best to you, Ken