I don't have the answer. I just started looking into Gen III stuff.

I think if you are working on an older Mopar, Then a stroked big block may be easiest because it will fit the stock K-member.
I think either option may be close in a hp per dollar for the engine, with many differences in fitment, accessorys and drive, and fuel system.

If you want modern EFI, coil near plug, serpentine belt drive, with high output alternator, A/C, and power steering, I think starting with the GenIII will save money, but fitting the engine to a old car and the computer/electronics may add to the costs?

A big block with carb, and minimal accessories likely will be less expensive, but converting the old big block to modern accessories and port EFI can be expensive.

In my case, I overspent converting the 505" stroked 440 in the convertible to port EFI and Billet Specialities Tru-Track.
The Tru-Track serpentine belt system was over $3,000 plus all the lines, remote reservoir(s), and other stuff likely an additional $500?
The Edelbrock XT EFI was about $4,000 and I ended changing out the ECU, fuel pump and such to about all I really used with the intake, TB, and sensors. The total cost got out of control likely an additional $3,000 on top of that for the FAST XFI 2.0, dual sync dist, wiring, EFI fuel tank and such.

On the other hand, I picked up a 80,000 mile 2008 6.1L Hemi with NAG trans off E-Bay for $4,000 with all the accessories, manifolds, stock wiring harness and computer. I think I spent $200 on a new electronic accelerator pedal and transmission shifter. I haven't decided on what to do with the electronics yet as more options are starting to come out. Looking at the Holley Dominator EFI?
The conversion K-Members are a bit pricy. The RMS alterKtion K-member / handling suspension kits are around $5,300+ brakes at $700 to $2,000.