Grant,
If you cant find 261 pistons there is another option. You can use Buick 181 V6 pistons (3.800" standard) adapted to GMC rods. The downside is that there is a bit of machining involved and a lot of careful measuring. There seems to be several different compression heights and style of piston for the Buick so choose carefully. Once it's done it is a better setup than the original and you will have a better selection of rings, higher compression, longer and stronger rods, and maybe forged pistons.
