A 351W can be mounted into a Mustang II. It is a daunting task but a lot of fun. There are many considerations and I will run down the list of items I can think of. My setup uses a C4 Automatic Transmission, a front mounted block plate with the side motor mounts removed. Also used were 351W Drag Race Swap Headers (Hedman, last I checked were no longer made).
1) Decide what you are doing for headers - a) use stock 302 Manifolds, no motor mount options needed. b) Use headers This is harder, the 302 sits offset in the Mustang II by about 2 inches to the passenger side. Thus you need to center the motor to make headers fit. I used the 351W swap headers, all 8 tubes drop under the A - Arms making it a must to also use a block plate. (A block plate is a 1/4 inch sheet of heavy steel that mount the motor to the frame from the front, instead of the conventional motor mounts.)
2) The block plate will center the engine when you mount it, BUT remember your transmission must be centered also. This is easily done by measuring and re-drilling the mount holes for the transmission mount. The driveline offset is fine, no adjustments needed there. Lastly you can use the stock torque converter and bell housing but you will have to "size" the oil pan (more on that later).
3) Block Plate issues: The block plate requires using an electric fuel pump and modifying the pump block off plate. One bolt holds the modified block off plate fine as shown below:
as you can see my oil gauge line has leaked over the years but not the block off plate, simply cut the bolt hole off. I have heard of mention that you must use a remote oil filter setup with these headers, I have not done so but use a shorty filter that fits the 351W Engine instead of a remote setup. (Pennzoil PZ-106)
Mounting the block plate: The plate mounts through your timing cover. You can use your 302 timing cover it works with the 351W. The plate I bought was a generic small block Ford plate. It came with 4 L brackets, bolts, and rubber mount discs. When I first mounted it the Harmonic Balancer hole was too small, I had to have a machine shop enlarge the hole for the 351W. Then I had them weld the tabs onto the plate, and I welded the other tabs to the frame rails. The rubber disc sits between them and the bolt goes through the center.
Power Steering and Alternator: I Removed the power steering from mine, however a custom setup can be done to keep it but you have to custom build mounts for it, and you would have to modify the plate as well to accommodate the pump. The alternator can be mounted using the original mounts with slight modifications.
Radiator: The stock 302 will not suffice at all for the bigger 351W you must use a custom build radiator or modify a generic one. Ron Davis Radiators has the Template for the II radiator. It fits perfectly with the OEM Shroud. *One note I have an electric fan on the front as well.
Oil Pan: You have to buy a front sump 351W Oil Pan, late 70's early 80's ford trucks used them. They are too fat too fit the starter with the original II bell housing. To solve this mount it, then using a heavy sledge 5 pounds or more, pound the pan in where the starter goes. (I know the purists are screaming, I did too, but it works, the other option is a custom pan at $600 bucks) In any case keep massaging the pan with the hammer until the start will mount flush as it should.
Hood Clearance: Nope it aint gonna fit without a hood scoop. Find your favorite style and cut a hole in the hood with a metal cutting jigsaw. (see below) If you want the same cowl style I have send me an email and I'll tell you where to get it and how to make it work.
Ground Clearance: IMPORTANT! The combination of adding the extra weight of the 351W and the tubes dropping UNDER the A-Arms makes for a very low front end and an absolutely header crushing experience. Unless you are truly ONLY drag racing you need to replace the front springs with a heavier load rate springs, in addition I would recommend also making the spring 1/2 taller as well. I also replaced the rear springs with stiffer ride and taller shocks to balance the look. In the end the ride looks good, a little bit gasser'ish, some may say 4x4 but it looks like this:
If you don't like this look DO NOT go with the under A-Arm Headers.
That's all I can think of at the moment... now get the 351W in there and the hotties will come flocking. :)