bigjbear almost described my 75 perfectly.. 350,sm465,205, D44/12b All stock. got mine relativly unmolested (save for the giant hole in the oil pan!). But as far as the truck was when I got it, it was not a frankenstien of parts from other trucks. My point is original un-molested 73-75's are out there, there are just not as many. Look on EBay as there usually are a couple that pop up every couple of weeks.
Not knowing Cali, I wouldn't know of a shop but do remember seeing something in Peterson's about a shop called GM Truck Center in Southern cali. Burbank maybe? I know they built a sweet full convertible K5 that was featured in either 4WOR or Fourwheeler, just can't remember.
One thing you may want to consider is the high swapability of parts on these trucks. 1/2 ton to 3/4 ton axles are a bolt in, so if somebody already did the swap as long as they used parts from another GM truck a good portion of work would be already done. Just make sure the axles are not wiped out/bent or in poor state of repair, but you should do that on any truck you look at, right? Remember any small block bolts in, big blocks too. plus later (86/7 and up) have one piece rear main seals and factory roller lifter cams, so newer is not always a bad thing. Shoot for that matter the sheetmetal interchages too. Alowing you to bolt on a 91 front clip to update the look and still have the truck registered as a 73-75 and not deal with smog. It's the best of both worlds then.
Bottom line is just because a truck has had some upgrades made, does not always make it a frankenstien. If done right it will show. Done wrong should also show up right away when inspecting it. Plus buying somebody else's project that they grew tired of or are in need of money (again built right) can save you money in the long run. I've seen a few trucks for sale here that the guy didn't get nearly what the sum of the parts was worth but sold it anyway. Some guys part them out and still don't make up what they spent. Crazy but it happens...