fattest cam in a 340?

One of the big issues with slapping a large cam in a stock 340 (other than turning it into a dog, because parts don't match up to the build), is Retainer to Guide clearance.

Around .510 - .519 lift your valve spring retainers will probably hit the stock guides & shred the valve seals. I've measured the clearance on several stock 340's and it was always in the .510-519 range max.

Any more lift beyond .500ish (give or take, varies from engine to engine) requires the guides to be cut/machined down and the use of Positive Fit Valve Seals.

I've had 2 340's and 1 318 (with J heads on it) where I could fit a Lunati .494/513 cam in, without having to machine the guides down for clearance. But that was unusual.