What you're doing is making your truck original. The reason it's difficult is that you're not following someone else's cookie cutter recipe. Be prepared to do some work to bring the vision to life.
I posted pictures of an early '90s Toyota truck wheel in your original wheel / disc brake thread. It appears the center of the Toyota wheel, the "hubcap mounting surface," is larger than the center of the stock GM wheel. For me the next question is whether or not the OD of the GMC hubcap at the shiny part is large enough to look good on the Yota wheel. If so it might be possible to make some nice inserts for the wheels that would allow you to use stock hubcaps.