I think it's a solid starting point, and with it's nice thick walls, i'd leave it at 100mm. Believe it or not, I have plans on file for a 100mm biral, as we figured that was the so-called limit we thought should be imposed on such a cylinder. I figure something in the 150 HP tune should live 100,000 miles, if done right with those cylinders. Bump it up to 180ish, and you might get 50,000 miles, but with increased maintainence and slightly reduced reliability. For longevity, maybe something in the 100 x 74 range would be good, but a 100 x 71 sounds just as appealing to me, since with the right cam, it will make good torque and be both cheap to build and will last over 100,000 miles in proper tune. What kind of cooling system are you planning on running?
Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance