If you go to Jolly's GMC page in the links section, you'll find some specs on the Clark 5 speeds. Problems with them are cost, weight, lack of spare parts, slow shifting and cost. You're talking over $1,000 for a 5 speed overdrive with synchros. Either way, you will have to lose the torque tube, best done with a differential swap at the same time.
Personally, I would prefer a Muncie M-20,21 or 22, they are stronger. But my friend has a saginaw 4 speed behind his 261 and it's been strong enough for him. Even has had the tires smoking into 4th gear.
