At 6-8" extension, you do not have room to install the shank in the "extended" part if your intent is to maintain 2.5".
What I did, was buy a 24" long Curt "weldable receiver tube", stuck it in my 2.5" receiver at the extension I needed (was about 12"), and marked the pin hole. Drilled on the drill press. Welded 15,000lb rated D-rings on it for the safety chains (looks totally inappropriate when viewed from above, if you get my drift). Made up short safety chains to chain the extension to the receiver, which you probably won't need for only 6-8". Painted with black epoxy appliance enamel.
I use a standard 2" shank in the extension, and the extension goes in the 2-1/2" receiver.
Another option presented to me was Brophy makes/made an extended shank, about 6" longer than a standard shank. BH-20 was the part number for the 2" version.
Putting 10-ply tires on half ton trucks since aught-four.