I had Dave mod my 71' SLP a couple of years back and while I still love it, there's some things on there I could live without.
If I were you, I do the following:
BE/HBE and loop. If it has four speaker jacks on the back like mine did, two could be used for the S/R and one could be used for a RTN Level knob if you make the channels foot swtichable.
C45, SA, Fat and VH are all fun but they're just different flavors. Out of the four, the two I use most are VH (Voice now) and SAT. C45 is cool as everything sounds bigger and Fat is wicked nice even with humbuckers.
If you want to do switches, maybe Dave can replace some pots with push pull pots so you don't have to drill.