The heart of the (Gen 3) beast?
The OEM flatplane crankshaft from the Shelby GT350 Mustang. Ford part number GR3Z-6303-B ($1200 USD)
IIRC, the extra 0.2L is done via a bigger bore using spray-on cylinder liners which you wouldn't want in a racing engine, so the crank should be a direct fit into a standard Coyote engine with standard iron liners and give 5L swept capacity.
It would be a shame for all Gen 3 engines to be boring old lumpy crossplane V8s.
When they could otherwise sound like this:
https://youtu.be/zkgG6OzGBEc?t=79
The suggestion for Gen 3 is alloy block engines, so could we be seeing 5L Voodoo-derived Coyote motors against 5L alloy block LS (or LT) motors? There are plenty of 5L LS engine variants (perhaps more so the iron-block pickup versions) which could donate their affordable OEM crankshafts.
A lower 600hp limit should allow the use of OE crank bearings and so on, without the big dollar needle roller bearings used in the current engines.
Would Supercars be keen on allowing direct injection (and hence the LT rather than the LS series Chevrolet)?
These two new motors would be a good opportunity to up the compression ratio rule (say to 11.5:1 or 12:1, to make it easier to make the power) and up the rev limit rule (say to 8000rpm) for better sound. Afterall the stock Voodoo V8 revs to 8200rpm in a road car, so that's easily within the capabilities of the motor & stock conrods/valvetrain.