I need help with Magnum Edelbrock heads.... Which valves?

You should remove an undamaged valve and measure it.

As supplied from Edelbrock, the assembled SBM LA head comes with intake valves that are 4.980” long, and ex valves that are 5.00” long.

The Magnum heads, according to the Edelbrock catalog use different valves, with the part number for them listed as
“Call tech”.

Measure what you have and see what’s available that will work.