I got a 400 based 451 in my '65 Barracuda. I used Streetmasters mounts, sounds like the hinged mounts the others talked about, allows you to rock the motor a little to the passenger side to clear the steering shaft. Proparts headers from Bob Mazollini (SP?) are the closest headers you'll find that'll fit without lots of work with the exception of the angel wing style Hedman Hussler that cost over 700 bucks. Just a few mods to 2 pipes around the pitman arm and the pass wheel well (take it in about 2 inches) Clearance under the car is lousy but a stock height front end would help alot, mines lowered about 3 inches. This car is a project on the back burner for months so no wisecracks about the spiderwebs! Infants will do that to ya.
440 would give you more ground clearance but would suffer up top, although it may help a tad over the shock mounts. the Proparts hit my pan rail on my 400 so I had to ding them. Pound flat the trans hump seam, install 3/4 inch spacers between the K and the subframe and dont run power brakes or a 4 speed ( I dont know if the z bar would fit down there). Hi-po manifolds will fit great, a very smart option for a torque motor as the exhaust headache will all but be solved, just mill the driver side manifold about 5 degrees on the mounting surface so it tucks in a little better and the steering shaft will clear with ease, may have to modify the joint to a "flaming river" type coupler rather than an OEM piece, and forget power steering. Now to just add the snout bars and tie them to my shock mounts...