im trying to choose a cam for a 350 im building and ive heard that stock chevy heads have a somewhat limited valve lift. my head casting numbers are 3970126. does anyone know how to figure out the valve lift or know what the lift for these heads is? also how do i increase valve lift.

Typical stock lift with factory springs, rockers etc is roughly limited to around .480 lift. There are a lot of factors which limit the amount of lift.

Compressed springs at max lift should have at least .060" between the coils. It's always recommended to install the proper valve springs for the cam.

The slot in the stamped steel rockers may hit the rocker stud at max lift. Long slot or roller rockers are needed.

The distance between the top of the valve guide and the very bottom of the spring retainer must be greater than the maximum lift.

Piston to valve clearance should be checked with very high lift cams.

You can increase valve lift by changing the camshaft or by changing the rockers to a 1.6 ratio instead of the factory 1.5. It will give you a little more lift but isn't a huge power increase. If you have 1.5 roller rockers and replace them with 1.6 roller rockers, you may see 5 more HP. You'll see a bit more if swapping from factory stamped steel rockers.

With the valve spring removed, you can dummy up the retainer and locks and measure the distance between the bottom of the retainer and the top of the guide boss and/or valve seal with a caliper. That will give you your max lift.

i think ill buy a comp cams cam set that comes with the right springs, retainers, and new locks and the cam chain and gears

Be mindful, despite all that, if the lift of the cam is large enough, you'll still have to mill the guide bosses for the lift.