Well the '74 models weren't set up for a V8, so their radiator support is further back, so a lot of guys cut it for clearance and mount the radiator on the front side of it instead of the back, which will mean using hood pins and removing the center support brackets for the grille. You'll need V8 frame mounts and motor mounts, which are very difficult and very expensive to come by (prices for full (USED) sets tend to start at over $150 on Ebay because they are out of production and Mustang II guys tend to horde them due to their rarity, since the few that can be found tend to be well-worn from usage and age by now.) The 1974 cars don't have all of the bracing that the factory V8 cars built from 1975-78 have in the chassis, their front springs are a bit softer, and some '74s had 6.25" rear ends, nowhere near adequate for a V8 (though quite a few of them, even 4-bangers, had 8" rear ends which are good for about 300-350hp depending on who you ask). The driveshaft and transmission crossmember is different, Mustang II V8 exhaust manifolds, starters, bellhousings, flywheels, flexplates, and clutches are all application-specific (and manual transmission flywheels are as hard to find as motor mounts at this point.)
Not trying to scare you away from it, I'm putting a 302 into mine VERY soon I finally scrounged most of the parts for the swap, even have a couple of "just in case" extras.
The 351W does add a small degree of extra difficulty in a II though, even using stock 302 Mustang II exhaust manifolds you'll either have to grind down the ribs on the passenger side manifold or slightly "massage" that side's fender well with a sledgehammer to fit the extra width of a 351, and you'll have to run a low-rise intake to clear the hood with the extra deck height.
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
1974 Mustang II Values 
