how much lift out of stock heads?

[porkchop]

how much lift can my stock 351C 4V heads handle?

 

[D.Hearne]

If you stick with a cam of no greater than about a .525 lift you should be OK. anything bigger and you need to check the clearances before startup.

 

[Cantedvalve]

how much lift can my stock 351C 4V heads handle?

Springs in those heads arent all that. I would say you are getting to the threshhold towards .500 Once you get a cam in there that needs a good amount of spring, you are gonna want to look at valves, retainers, and locks as well (multi-groove valves are just plain icky).

Are they rebuilt stock or just stock stock?