The engines you guys are suggesting are wayy too modern and complicated. Overhead valves are far too needlessly complicated. If you want a really compact engine that is able to be mounted super low, and makes a lot of torque while being reliable, I think a flathead is the way to go:
The advantages far outweigh the detriments. They made these engines for over 50 years, that means they are really good. Also, because it's older technology, by default, that means its better, more advanced (really) and way more reliable than anything that has come along since. Did I mention they are cheap? This is really the best measurement of a good engine, so they are the best, really.
I would really suggest a steam engine, and I'm currently scouring the internet to find one that would be a suitable application for a foxbody mustang. I will update this post as soon as I can. You might have to modify the firewall to fit the boiler, and the pushrods. Steam engines are largely superior to internal combustion engines because they produce far more torque. Nothing close as far as horsepower, but this thread obviously and substantially proves that heavy engines, with older and superior technology, with lots of torque are clearly the most superior type. Again, I'm still trying to find information, but I really think steam is the way to go.
Edit:
Good news. Although I couldn't find much useful info on the internet, I did remember that my dad had some old steam engineering books. I found one that looks promising. It was published in 1896, so the engine designs are probably incredible. Will update soon.