Can I "borrow" rocker arms from my '65 273 for my '71 340?

66fs said
The rocker ratio is still the same, but the motion of the pushrod is different. Think of it like jointed pushrod. The farther down the adjuster ball is from the rocker, more it swings away from rocker shaft centerline at max lift.

Yup, and remember if making the adjuster longer or shorter moves it closer to, or further from the shaft, it is affecting ratio.