This notion sprang out of M.A.D's concept for overwing nacelles on the Hercules. I've since had second (third?) thoughts about those overwing nacelles. Mostly it came from looking at other re-engining jobs on turboprop transports. It seems to me that - in constrast with Antonov's An-32 - the commonest solution to big increases in shaft horsepower is props with more blades. [1]
One example is the Aeritalia G.222L with (albeit derated) Tynes in place of T64s. Going from 3-bladed HamStans to 4-bladed BAe propellers was all that was needed. (Although, I must admit I love the idea of full-powered Tynes mounted over the wings of a stretched G.222!)
With the Herc itself, multi-blade props handle the C-130's more powerful AE 2100s. And, at the more extreme end, Marshall's 'Snoopy' testbed could mount a huge Europrop TP400 amongst its T56s thanks to the former's 8-bladed sickle props. Below is a retouch of 'Snoopy' showing the revised TP400 nacelle duplicated (with the outboard nacelles removed) to show what I mean.
From that, I conclude that the obvious approach to a TP400-Hercules would be a twin-engined 'Giant Spartan' >;
Two TP400-D6s would give this re-engined Herc a total power of 22,000 hp (versus 18,550 for the C-130J's four AE 2100s). A downside is a slight weight increase - 8,333 lbs total dry engine weight for the two TP400s vs. 6,910 lbs for four AE 2100s (or 7,760 lbs for four T56s). On the other hand, with the TP400-Herc, all that iron mongery is being carried closer in. Dunno if that's much of an advantage.
Anyway, the big selling point for such a conversion would be engine commonality for A400M users who also operate Hercules. Stealing a suggestion by Logan, such re-engining would make most sense when combined with a wing box replacement programme. New engine bearer points and wing strengthening could all be incorporated into new centre sections.
______________________________
[1] Other than re-using existing nacelles, this approach has obvious advantages for maintenance. Engine changes are greatly simplifed compared with the Clank. Even performing simple mtx on An-32 engines can't be for techs with any fear of heights!