I'd head over to your favorite automotive wrecking yard and start scrounging thru the GMC section.
Take your old blower motor with you, and see which GM product's HVAC blower matches your bolt pattern. Usually on the engine compartment firewall, passenger side, easily accessed.
I pulled one off a mid-'90s Buick awhile back and installed it in a friend's '63 Corvair. Bolted right in.
GM did a lot of this kind of stuff, you should find success for little $$.
FWIW & HTH. . .