I recommended the Explorer engine for a couple of reasons. One, because the one piece rear main seal in the later blocks is far less prone to leakage than the rope style seal found in the pre-'83 engines and the later roller block utilizes a roller camshaft design that has proven much more smoothly and efficient than their earlier flat tappet camshaft design....not to mention the roller engine will run forever.
Another reason I recommended the Explorer was because the Explorer engines came with GT40 and GT40P head castings. These were probably the best factory iron head offered on any small block built after the muscle car era. They're light years better than the small, restrictive emissions choked units your Mustang II's original engine would have come with and still even far better than the E7TE castings the later HO engines were saddled with.
Explorer engines can also often be picked up fairly cheaply compared to an HO found in any Mustang, Lincoln or Thunderbird. The only thing you really need to do to the Explorer long block is swap out the camshaft and valve springs and you're golden. Not to mention....if you buy one complete, you should have no trouble selling the Explorers upper and lower EFI intake to finance a decent 4-barrel intake for your carburetor.
My only caveat before pulling the trigger on this plan is to ENSURE that you can source a decent set of headers for gt40 heads and a Mustang II. From what I understand, it's hard enough already to find headers for a regularly configured head.
It's not to say that they don't exist. Just be sure before you run out and buy a wrecked Explorer.