I did just that on my '78 GMC PU years ago. Unfortunatly, the wires will have to be cut and re-spliced onto the new CD players plug. I had to cut the opening for the new player, which I used a sawsall for. It could be easy to cut too much with that kind of saw, but it could also be done with a hacksaw blade, by hand. If you're not into doing the wire work, a local stereo shop could do it very easily.
Yes, more than likely you'll have to splice in. Not too bad though since there are only about 11 wires to hook up, 8 of which will be speaker wires if you have 4 speakers in it. They are the easy ones to find since each speaker will have a solid wire and another of the same color but just with a stripe to differetiate between pos and neg. For the other ones you'll have a ground, accessory power, and constant power. As far as cutting out the dash, I used the metal sleeve that comes with the cd player and used it as the template for marking the metal dash plate and plastic dash cover. I usually use my dremmel to do the cutting.
I don't know for sure if they're available for such an old truck. Also, most new stereos have a wiring diagram on the back, or at least tell you what color wire does what. If your stereo doesn't have that, you might be able to find it on their website.
Hooking up a stereo is one of the easiest things you can do.
When you spliced into the red power wire, you somehow spliced into the headlight wire. Get behind there and with a multimeter look for a red wire with around 12vdc on it when the ignition is set to run and acc.