Is the engine a small block Chevy? Those PS brackets and pumps were pretty much the same from the 1960's through the late '80's on most SBC-powered cars and trucks.
Imo try the classified ads, such as Craigslist and find someone who is parting-out an appropriate Chevy car or truck or GMC truck, and have a look. Chances are you can pick up the bracket for a couple of bucks.
Alternately, you may be able to drill, heat-up-and-bend or modify the existing brakcet.