Enjuku already has you covered...
S13 SR into S13 chassis
Wiring Specialties S13 SR20DET into S13 240sx Harness Combo (Upp [ S13S13_Wiring ] - Enjuku Racing - Performance Parts and High Quality Fabrication
350$
S13 SR into S14 chassis
Wiring Specialties S13 SR20DET into S14 240sx Engine Swap Harnes [ WSP-S13S14-MAIN ] - Enjuku Racing - Performance Parts and High Quality Fabrication
320$
Cant beat that, new harness, already converted.
There really isnt that much of a market for it... People will have their friends do it, or they will buy a premade like that...
For the money that you would have in quality wire/sheething/connectors, and then your time, you wouldnt make that much $$