1) Are the Dakota and Durango frames similar as far as being a donor for a '40s-'50s truck?

2) Do all Magnums share the same mounting position? Basically, if my donor had a 3.9 Magnum can I physically put a 5.2/5.9/4.7 in its place?

No not all the frames are the same, the best ones for this swap are the 87-96 Dakotas. The 87-96 are narrower and take the five lug axels and rotors.

If you have a good frame I would just mustang II it. My frame was hacked beyond repair or I would of mustang II it.

2wd and 4wd are different it is coil 2w vs torsion bar 4w-----early 2W frames are better because you can use the 5 on 4.5 hubs and you can buy 2 inch lowering spindles and if you need it lower you can also get 2 inch lowering springs....bob

I found some really good info on Allpar regarding the differences too. I looked at a Durango donor yesterday at the auction. The frame bows out right at the point where you'd want to shorten it. Bummer because Durangos are cheaper than Dakotas around here.

I believe all Durangos are 4x4. Pretty limited use in a street rod. Gene