1. A 302 will fit for sure in your Maverick or Comet, and will look nice and cozy in the engine compartment, like it belongs there (because it does). Check to see if the engine going in has a forward or rear oil sump. The Maverick requires a forward one, and you might need to get a new oil pickup and pan if your case is the opposite.
2. An engine crossmember from another 8 cyl Maverick or Comet would add a little chassis rigidity. Consider finding one of those in addition to the frame mounts, and tranny crossmember.
3. For ease of installation, I too recommend using his A/C pump, power steering pump and alternator. All the brackets are already there. P/S pumps are P/S pumps, and if it works, that's all that matters. Alternator amperage, too, varies little amongst these Fords. You might have to relocate/replace the windshield washer reservoir, depending on how the bracketry holds your accessories in your engine compartment. No biggie.
4. With the power steering pump, you will have to fabricate a new P/S hose that has the LTD P/S pump connector on the top end, and the Maverick fitting on the bottom end. I took both hoses to a local hydraulics shop and they made short end of it.
5. The shock towers in the Maverick/Comet prevent you from using his exhaust manifolds. I purchased exhaust headers from Summit for a Maverick and had to do some major mods with them:
A. The steering box on a 250 P/S Mav is directly in the way with one of the pipes. I had to pound it to almost half its diameter to get it to fit.
B. The P/S assist cylinder at the bottom also interferes with the headers. I had to pound the headers, and had to make sure that the tranny crossmember was tight enough in the tunnel to hold the headers off of the P/S assist cylinder. Consider removing the power steering--I kept mine, but I thought it was more hassle than it was really worth.
C. The headers hang off the bottom of the car with 3" of ground clearance, making speedbumps a real hassle. Consider modifying the headers to fit up tighter under the car (one of my next projects).
D. The shock towers had to be trimmed a little in my application since I put the headers in after putting the engine in. Consider bolting the headers on first before lowering the engine into the car, for ease of header mods.
6. The throttle cable from the 250 will work on the 302, but is considerably longer. After a while I would consider finding a shorter one to increase reliability. The best ones come from other 302 Mavericks, but a Fairmont/Zephyr one will work with minor mods.
7. The three-core radiator will work just fine with the new 302. You'll want a new 302-spec radiator cap (just to be safe) and the 302's recommended thermostat. I find that I run just a tad hot, though, so one of my next projects is a BeCool radiator and separate tranny cooler. If the LTD doesn't come with a coolant reservoir, then I recommend finding one, since my Mav spits out coolant at high RPMs--probably because of minor air bubbles in my system.
8. If the engine you're putting in is of the serpentine-belt type, then you'll have a counter-rotation water pump. This may not work with your 250 radiator because the water outlet of the water pump may be on the wrong side for the radiator, and the heater hoses may also be on the wrong side. If this is the case, consider an electric water pump and fan, or figure out some way to be really clever about routing your hoses. If you have the V-belt variety, then no problem.