Any GMC roller rocker arms for sale out there.

Nothing I'm aware of, Jesel etc. supposedly can make them but $$$.
Nothing adapts - way too long between the adjuster and roller.

You could try getting hold of the Fergusons (current Inline members). They made their own 12 port heads and accessories including roller rocker arms.

Unless you can find some used. Just about anything beyond stock replacement parts for these older engines is all custom made by you or someone you pay to make them. There was never that much demand for those racier high performance components for manufacturers to offer them. Try Harland Sharp also, you can buy the roller tips and axles from them and the adjusters.

The later stamped steel stock rockers are pretty tough in an old time hot build. How they hold up with todays extreme builds is another question.