[hatzie]

That's probably the U63 Delco 41APB1 radio GM used in base trucks and some of the cars in '74.
Radio service manuals for the 1970's & 1980's radios are very hard to come by. This was right on the edge of the truly throw-away radios. The guys that did this kind of service work tend to hang onto their paper manuals and keep them "in the family" of radio enthusiasts. It's almost blasphemy to let this information outside the community.
Repair of even non working units is not that difficult but re-aligning the receiver coils, if they need it, is an art and not that easy without some guidance.
That being said they do show up with more regularity on Fleabay than they used to... Here's a SAMS Photofact manual for the 1974 AM/FM radios. It may be close enough if you're not frightened of older electronics. http://www.ebay.com/itm/1974-1975-CH...28488685&rt=nc
EDIT: I found a 1974 Delco all GM models radio manual and bought it. I'll scan it and post it up in the manuals thread. Probably after the end of the month. If you want the paper original once I'm done we can make a deal. I've been on the lookout for these for some time and this one popped up at a reasonably inexpensive shipped price. Usually the seller seems to think they're printed with platinum ink on gold plates.