Jim has it covered all covered.

You only need the 3rd wire mod on the neck pickup, as you only need to invert one pickup to put it out of phase with the other (and the neck is the easiest option to mod). Two pickup series/parallel switching on a 4 or 5 position switch needs at least two poles and individual lugs for each position on each pole. Hence, the use of the Grigsby 4-way (or the 6-lug per pole Grigsby 2 and 4 pole 5-ways). The ‘import’ 7/8 lug 5-ways link/short two lugs on each pole to get the 2 and 4 positions, so can’t be used for independent series/parallel switching.

A 3-way ‘import’ or CRL style switch combined with a push/pull for phase and another push/pull for series/parallel could be made to work, as the 3-way doesn’t short across lugs for the 3 positions. But, it means a lot of switching if you want to go from one sound to another. The easiest solution, IMO, is to use a Grigsby style switch and mod the cavity so it’ll fit cleanly. Or, possibly, a Freeway style toggle... but, expensive.