Thanks to everyone that helped me find the marriage registration!
I looked at scans of book 663.2 at GeldersArchief; but did not find it.
I asked Streekarchief about source for their index. They told me it was book 22; which is book 663.2 at GeldersArchief!
Comparing the index to the scans; I found...
SCAN 663.2-0015 last item matches this index item:
Camphuis, Reijer, ;(onder-)/trouwt op 26 06 1778/19 07 1778 ;in Epe ;met Jacobje Lamberts.
SCAN 663.2-0016 first item matches this index item:
Pikeren, Hendrik Gerrits, ;(onder-)/trouwt op 10 09 1779/10 10 1779 ;in Epe ;met Engeltjen Henricus Ruijs.
Perhaps some pages from the marriage book were not scanned by GeldersArchief.
GeldersArchief have added this to their list of things to look into later.
Streekarchief have scanned the registration; which i uploaded here:
https://www.werelate.org/wiki/Family:Aalt_Westhuis_and_Mechteldje_Cornelissen_%281%29
Thanks again to all those that helped me find this item and also the additional information provided!
Frank