what is largest cam for 340 with factory 10.5 flattops????????

.509 drops in fine, never had preoal issues.ime
The .528 is way better/durable cam though, broader range,better idle and around 10''-12'' of manifold vac -roughly- it was the modern max wedge grind 2800-6900 and will actually take you into the 7200 range if you install it straight up 112 or 110*
I run mine 107* installed with the 4spd/3.73 gears, good cam

Also the .509 kills the cyl pressure, the .528 with only around 60* o lap doesnt hurt it as bad. gotta remember the ''10.5'' is usually 9.8 or 9.5 comp with the loose fac tolereances.

Always check for guide retainer interference, but about .520 lift is what I have found to be max without cutting down guides... that is with a solid lashed .028/.032 lol

measure it, JMO