321" are .125 over bore from stock 4". Ross has pistons for GMCs. Arias takes a few weeks. If you offset grind the crank for the stroke increase and use the stock rod length of 7" the pin can move up 1.25 to help with piston weight. I have heard others using 300" Ford rods but I don't know their large end diameter or length. If they are smaller you may not need to weld up the crank.
You can call Doug Rodinson at Horsepower Engineering on Fulton St. in Pasadena Ca. I believe he used an off the shelf Oliver rod and off set grind for of his GMC's...Good Luck
