This is a
good thread to look through if you have not found it already.
A sampling of 1967-68 Fremont trucks in the thread all have seven digit RPO codes that started with "04", and might have looked like the below.
04D32AA LH SRT RV MIRROR
04Z52AA FOAM SEAT
04M20CJ 4 SPD TRANSMISSION
04C42AA DELUXE AIR HEATER
04D36AB NON GLARE MIRROR
04760BA HVY DTY BATT
04600AB BLACK VINYL TRIM
I'm not sure if this is helpful: for example, there are a couple distinct codes for the M20 transmission. Given that your GMC may have never had a SPID at all I am not sure how exact you want to be.