Its about time to put new fenders on my blazer, I was wondering if the 81-87 fenders would be a direct bolt on, or if I have to swap everything, (hood, cowl, radiator support) If anyone has done this let me know.
I've gone the other way - used my '75 front clip on an '81 cab - but to do what you're thinking I believe the following will be required:
First off, the entire front clip that you mention - rad support, hood, cowl etc.
Second, because the hood hinges are different it will be necessary to cut a slot in the cowl area of your '74 cab to accomodate them. The picture shows the cowl area on the '81 cab I am using for my K-35 project. Also note that the hood hinges bolt to the cab, not the fender. What isn't visible (because I have already welded them up) are the holes for the cowl piece to attach to the cab.
The wiper drive is also a little different - everything more or less bolts up the same but the wiper studs are longer on the 81 - 87 version. Naturally, the bolt holes aren't an exact match but you can make the wiper drive assembly work - in this thread is a bit more info on that, although again it deals with using the newer cab on the older style truck:
If you do want the newer front end on your Blazer, I would suggest getting a complete donor truck for the front clip and using it as a pattern to make the required modifcations to your Blazer body.
I have seen this done - a tow truck that got some frame work done in the welding shop recently had a '70's cab with mid - '80's front end. Now, they had done somewhat of a "butcher job" with the installation but it is possible.
So it's not an impossible task, just a bit of detailed work required. Good luck with the swap!