My Dart Swinger Project!
Very good question..... Magnums are not all that expensive around here (and I am about 100 mi NE of DF75, in the same general market region). ANd, Magnums have a bit higher CR to start with due to lower deck. Downside: any other used engine is another crapshoot as to condition.
I'd guess $125-150(???) to clean the block before and after, and bore one hole, and press on one piston. A single 526AP+.030 piston is showing up online for $25.
The nice part of going back to the drawing board and boring all them out is that things can be cleaned up and set right from the get go. But it's that nagging question of budget.