Very cool car. I just picked up a near twin to yours, just in different colors. You're absolutely right that there is little to no info on these cars available. I have been purchasing any and every Caballero brochure/book/etc on eBay just to get more insight on options and production. You indicated earlier that someone was quoted as saying GMC only made 60 Diablos a year. Where did you see that?
I'm also interested in knowing what route you go with your decals. I think my decals may be the wrong color and that when my car was repainted that person simply went to Phoenix Graphics and bought what was available. Phoenix only offers 3 color combos (Gold, Red, Blue) yet in 1978 there were actually 5 combos - Red/Black, Blue/Black, Gold/Gold, Gold/Black, Black/Gold. My fender tag is stamped "11U, 11L, 56M" so I would need decals that are outlined in Black with the center being Gold. Your decals are outlined in Gold with the Center being Black. Both of these options do not appear available through Phoenix. If you go the custom route I would be interested in knowing who you go with.