FYI, I own a bunch of 351 Cleveland and 400 parts.
The 400 Ford is a small block, it shares most dimensions with small blocks, virtually nothing with big blocks. Thus it is a small block, the same goes for a Cleveland. The 400 had one part of one year when it came with a bell housing pattern of a small block. I doubt that you will find those. So basically it takes a big block patterned trans.
The deck height of a 400 is over 10", around 10.2" so the exhaust manifolds will be extremely close to the fenders and AC box. I doubt that it would work. The heads are Cleveland heads, basically an identical head to the 351C 2V. I have two sets of 351C 4V heads, and one of the 2V heads. The engine mounts are unique but easy to get, they might very well bolt into the frame like other small blocks.
The engine length is identical to all small blocks, they take the same 4.0" pistons, lifters, Cleveland cams, timing chains, balancer. The timing cover is bigger, as is the intake.
I know the idea sounds good, but given the tall height/size, the 351W is a far better choice. I'd love to build and sell my 400 as planned and sell it. I have special intake plates to bolt on 351C 4V heads and a 351C Torker intake. I was doing it for my 73 Ranchero.
Do a 351 Windsor if you want the cubic inches. You can buy a stroker short block for about $1500, and all 302/351W heads bolt right on. The inexpensive $750 aftermarket aluminum heads available would be great for such an engine. With some custom parts to adapt the Explorer 302 PCM system to one, that motor could make an easy 400hp, in all conditions.
I'd love to see any combination, unfortunately the old 351/400M engines are not feasible any more. Good luck,