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I think what you'll be up against is that the market isn't large enough for a company to mass produce seats for a 35+ year old truck. I'd recommend asking in the '73-'79 forum to see what options there are for seats from other vehicles that would fit your truck. The other option is to talk to a custom upholstery shop in your area, but that option is likely to be higher cost.
I guess I'll look into having my seats cut down some. It's too bad all these companies selling seat for "trucks" are actually selling seats for vans, rvs and broncos.