Although GM small blocks have similar displacement to the modern Hemi, their valve configuration is in-line with the length of the block. The Hemi, however, has the valve placement, perpendicular to the block. This results in considerably greater width. You should take some time and measure everything very carefully. There's a strong possibility that it won't fit between the shock towers. Older mustangs, 65-70, that have 4-valve V-8 engines, usually have the shock towers removed and employ mustang II IFS. It absolutely can be done but it may take a fair amount of fab work.