and adapter plate can be made for each side of the falcon to mate up to the V6 mounts.
the best thing to do is to take a bare block and lower it down into the bay to see what needs done.
if you need to get an engine or block. most junkyards will have a RWD 3.8 ford. becareful though the FWD 3.8's use the old 2.8 bellhousing and are not a good choice due to lack of suitable trannies.
also has good quality running engines and a few blocks. ideally an 01 or 02 4.2 truck engine would be the best way to go. they are very torquey and come with a bottom end girdle and windage tray from the factory as do the 3.8 01 and up mustang engines.
you can always get a 4.2 from www.rpm-mustangs.com
i have dealt with them before and they are a first rate company with excellent customer service.
they have a 4.2 longblock for about $4000. that includes forged rods/pistons your choice in cam and i believe choice of roller rockers ratio (1.8 adjustable are the best)
here is a direct link http://rpm-mustangs.com/cart/home.php?cat=281