You don't have to change steering box or use headers. I have a 55 Crown that has a 1972 351C with stock 2v cast manifolds and stock 55 steering box.
I also started the post about modifying SBF Butch's mounts to install a 351c in a 57. It has 77 pickup 400M manifolds which are larger than 351c 2v manifolds and still fit very good in the 57 chassis, but idk if those manifolds would fit in a 56.
Butch's mounts set my 351c too high in 57 chassis, as did the stock 71-73 Mustang 351c mounts and the later 84-93 Mustang 5.0 mounts appear too high also, though I have not tried them as yet. By "too high" I mean you can put a clenched fist between the oil pan and engine crossmember. Those mounts appear to work better for the 5.0 though judging from other installs on 54-56 chassis. Maybe cuz different pan profiles...
63-64 Galaxie SBF mounts, 70's SBF pickup, 77-78 LTD SBF mounts all sit too low (oil pan sits on crossmember w/o spacers added), but some people are using them with 1/2" to 1" shims between the mount and the motor.
Some of those mounts are short enough to use a fabbed adapter plate to put between the frame mount instead and the later SBF mounts mentioned and adapter fabrication would depend on which motor mount you chose (studs or ear-type crossbolted mounts). But I prefer the shorter mounts cuz easier to add height than subtract it.
The 351c 2v manifolds clearly will work with several motor mount possibilities discussed in detail on those 2 other threads. If you have 351c 4v heads and manifolds it will be hard to find a lot of info on those and those may interfere with the steering gear cuz they are larger too.