I'm looking to add the sd1 pedal onboard to my gretsch 5122. I'd like it just before the output jack so I can still use the volume and tone controls o he guitar. ill drill extra holes for the pedals knobs. Does anyone know how to do this, and how to replace the on off switch with an spdt switch.
Have you looked at the MOD boards? they are built-in fx you install on your guitar, and in many cases the guitar stays stock looking with no extra holes.

They have a TS-9 type MOD board but not a SD-1, still worth looking at www.guitarfetish.com
if nothing else it might give you some ideas on how to do what you want.
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate