Here's my F350 build, might help with some of your questions. viewtopic.php?f=22&t=48572
For you C4 swap, you should be able to use any auto tranny from a Windsor smallblock V8 with your 300 but the 300 is internally balanced vs. external balance on the V8's. You could use a V8 flexplate (sandwich plate?) and just knock the wieght off or find and buy the correct plate. Here's some flexplate balance info: http://www.rowand.net/Shop/Tech/FordFly ... plates.htm
For the swap, you can reuse your pedals, just drop your clutch pedal to the floor and put it under the mat or buy the wider auto brake bedal and the pin that replaces the clutch pedal. Add some sheetmetal over the clutch rod hole (if you have one).
For you crossmember you should be able to reuse it and the tranny mount but you'll have to relocate the mounting holes on the crossmember or the chunk of angle the crossmember sits on in the taller F350 frame.
You'll have to find a driveshaft that will work or get your shortened and the C4 yoke swapped onto it.
For you rear to get a better gear ratio you'll have to swap it. The '70 Dana 70's weren't designed to run thicker gearsets. I swapped in a Ford Sterling 10.25" rear from a newer '89 F350 box truck (same width spring perches). It's pretty much a direct bolt in.
I wonder how hard it would be to find any of these trucks up through '79 or so down there with a C4 already in it. You should be able to pull most of the swap parts out and be much further along.