Bought a bare block to start a mock up.
The new block is a full 3 in. shorter than the 351C block. The dimensions on the PDF file shows that the engine length to the end of the crank pulley is also a full 3 in shorter.
This is ignoring the A/C I have no idea how the A/C fits into a mustang.
The new block shares the pattern of some bell-housing bolts.
Here I have it bolted up just for kicks.