Plenum volume on an EFI application is not super critical like it is in a carb application because you're not worried about providing the right vacuum signal to the carb. You can go bigger on the plenum on an efi build and not hurt anything.

The balance tube on the cross ram would be to help balance out vacuum signal to the carbs. Imagine at part throttle in a low/medium vacuum situation, the vacuum signal off 4 alternating cylinders instead of 8 will be more uneven. Distributor vacuum advance and pcv wouldn't oppose a more stable vacuum signal either! If the carbs weren't properly sync'd, I guess the balance tubes could help bandaid this as well.

If you're using a speed density EFI system, the balance tube would help provide a more stable MAP signal to the computer in much the same manner. You could downsize the tube size in an EFI application, however I would probably just leave it.

As for the intake runner length, rumor has it chrysler used a formula of 84000/runner length in inches = peak rpm of power boost. There's various calculators out there that use the speed of sound vs runner length to calculate harmonics, peak rpm ranges, etc. The ones I've tried do put the chrysler figure in the same ballpark.