The Mark IV big block V8 started life with multiple issues ... the biggest being it's like having four different V-twins stuck together. The 8100 made several improvements, but the most important was ignored. It still has a combustion chamber meant for a 4.47" bore slapped onto a 4.25" bore. The valves were tiny for its displacement. Now in 2019, there is technology to make a cohesive new engine, aimed at maximizing BSFC, lowering friction, and pumping losses. You can also find some efficiencies in leveraging the Gen V small block tech and parts. If you're going to reintroduce a big block, DO IT RIGHT. It could be your last chance!
I'd use a 111 mm bore and 105 mm stroke. This makes the best use of the bore spacing while still allowing cooling between cylinders. The shorter stroke simultaneously lowers the piston speed and attendant friction, and the longer rod also reduces friction. Since torque is proportional to displacement and volumetric efficiency, NOT stroke, this engine would make much more torque at the same rpm as the 8100, and with lower pumping losses it would get almost 20% better fuel economy.
A 8.1L needs 58.4mm intake valves to lower pumping loss, and a fairly large exhaust valve (46mm) to maximize torque at part throttle. Big ports don't hurt torque. 360 cfm intake and 270 cfm exhaust may sound high, but it's what a 496 needs. Raised ports and short cam duration will boost torque and response. Short ram intake also reduces pumping loss, reduces cross-talk (similar to a dual plane intake), and boosts low rpm torque by 15%. Like the Ford 7.3L V8, use port fuel injection with Denso UC injectors; it's cheap, quiet, and resist carbon build up. If you do stick with GDI in order to leverage the current EMU, go to 350bar.
The basic Gen V combustion system is pretty good, but the port between the pushrods was the "bad" port on the Mark IV. The 8100 actually got this right by putting the port outside the pushrod (too bad the port itself was horrible). Ideally you would use the NASCAR R07 valve layout. It optimizes in-cylinder mixture motion with ports between the pushrods, while being more compact than the Mark IV.
The 8100 should be brought into the small block fold and benefit from its technologies. Stretching a gen V V8 will make the 8100 only 2" longer. A cast iron block and aluminum heads would be marginally heavier than the 6.6L. Share LTX parts, like water and oil pumps, VVT, flex plate, 8 and 10 speed transmissions, and dynamic skip fire.
The sum of all this is a reasonable 425 bhp @ 4600 rpm, minimum of 525 lb-ft @ 3500 rpm, with at least 400 lb-ft just off idle, and approaching 20 mpg highway. The eventual crate engines will out power the ZZ572, on pump gas. It can also be bored and stroked beyond 9.0L.