FYI, one of the fellows in our group at Bonneville ran a stock appearing GL500 in Production Pushrod class. This bike has a 650 primary drive, mated to a 500 trans, 12.3:1 CP pistons, Webcam grind, GL650 valves, and Kemph ported heads. It also has the headers split and the inner tubing removed, then rewelded, plus a set of GL650 carbs. Density altitude on our time slips showed around 6000-7000 FT, and the rider weighs about 285. The bike would pull 11,000 RPM in the lower gears, but couldn't pull 5th gear.
The bike finally went 97mph on the salt....pretty good for the air density and rider weight! It is seriously hampered, for top speed, by the long intake and exhaust tuned lengths....it just WON'T come "on the cam" like my other engines do. We will try the bike with my carbs/manifolds and my exhaust lengths to see what happens at El Mirage (pretty similar air density, compared to Bonneville). I'll let you know if it works better.