First off let me welcome you to plowsite. There are a dedicated and knowledgeable group of people here who can help you with any questions you may have.
Now to your problem. I bought a western unit last year for my '85 GMC K-2500 and of course I was told it would fit fine and then I got to trying to put it on the truck and you know what I found out next was that I had been had. Oh well I was new to it and some old fart got over on me big time. Well live and learn.
When I tried to fit it up it was about 2" too wide for my frame. I built up the mounting location with 6" angle iron long enough to bolt to the frame and extend into the plow frame. I had to weld plate inside the angle to build it out to spec. It worked out fine and I had a solid year of use last year.
The bolt patern did not match up so the angle was made just long enough to bolt the back to the truck frame and still bolt the plow frame in front of the angle. In reality I have more steel inside of the plow frame than it had before and I did not alter either the truck frame or the plow frame I just made an adapter.
Barger Signs Services Div.
Sorry for the long post but there is a lot involved.
I would try to locate a used frame mount, or the pieces that are specific to your frame. I know plow mounts share many parts and often use just a few different pieces that are specific to all the vehicles that have the same front frame your truck does. My local plow dealer was very helpfull in locating used plow mount pieces for me and he gave me great prices on them too. I would advise against fabbing a mount on your own because unless you have the equipment and skill to weld the relatively thick steal used in the mount, it will break on you. Additionally, the manufacturers put atleast a little bit (i hope) of thought into how the mount attaches to the frame so as not to stress the frame or mount improperly.
Since Chev/GMC trucks of that period are so popular, I would suggest trying to get the proper parts as greasemonkey has described.
I HAVE built my own plow mounts, A-frames and other projects - but there is quite a bit involved in building these things. Since I weld for a living, it was fairly straightforward fabbing.
If you're prepared to build your own, you might find it more useful to take a look at some actual plow mounts up close & make sketches as required. Most pictures I have seen give you a good overall view of the complete plow but don't show many of the small details. Also, it would be hard to determined what thickness of material was used from a photo.
I have fabbed parts for plows before but usually I was copying a part off another truck or from a drawing for a manufaturer. I know Fisher has drawings. If you have the tools and means then it can be done. If not I would suggest searching. Look on ebay and in equipment classifieds.