Two or four of the studs are different lengths for the 350 and the 400. They are in the same locations, but are definitely different lengths. You will need to pull them from the 400 block if you are going to install the head. I'm not sure if there is a difference between the cylinders. The 350 also has a bolt or two that you won't use.
I would suggest just putting in a 400f complete engine instead of mixing and matching. That, or going with bigger pistons. I bought a set of the pistons (link shown in a previous post) from CMC (?) and they fit nicely. The machine shop I took the cylinders to was impressed with them too because they were made to tight tolerances. The only issue you will have is finding a shop that will bore and hone such small cylinders. If/when you find them, make sure they verify the pistons fit in the new holes.
I have a set of 0.75 over pistons/rings/clips if you want them. All told I paid $300 shipped for them. I'll let them go for $200.
Camelman