but seriously the '68-'72 Mustang box can be laid on it's side for push/pull steering ...another idea is if the springs/axle are out of whack which they probably are now is the time to get nice and low because the frame arches up at the front to accomodate the parallel leaf suspension if you were to put a late '30's ford front crossmember and spring in after you straighten the rails you will gain most of that arch in drop ...
I'm not even gonna comment on the lack of pics after thumbing through three pages of posts... I will say though, why don't you put a hemi in it and be done with it?
Ok let's get this back on track. The 37 Plymouth has a very long engine bay and the steering box sits on top of the fram and stick out quite far to the left. A flathead ford is quite short. A tape measure will tell you that with the fan in the correct location the exhaust will probably going to need to snake around the exhaust. So pull the hood sides and run lakes headers. Or I have a rebuilt 37 Plymouth 201 with a dual carb intake, header and milled head for sale. Which will eat that fords lunch.
I dig it! Performance parts for the inline flats are getting harder to come by while the flathead v8 while some are pricey, are definitely more readily available. I dont believe it would be that hard of a swap considering people have put 289s, 440s, and even old hemis into similar cars. I myself have a 1941 special deluxe, still not 100 percent sure the direction I wanna go, be it scrounge for performance parts for the inline 6, do a modern V8 swap, or an old school v8 like the ford Flathead v8. Ive already decided Im gonna do the explorer rear end swap and the t5 tranny swap as far as what i want under the hood is still undecided.