Altitude versus which? Mag or ground track?

Here's the real life scenario: The bearing (GPS track ) from Mojave
(KMHV) to Inyokern (KIYK) California is 006 Mag up along the east face of
the Sierra Nevada. Heading home the other day in VMC, the airmass spilling
off the Sierra was giving me a healthy quartering tail wind from the left
(port if you prefer) side. In order to maintain the GPS track line I was
holding in a crab angle of nearly ten degrees. A little arithmetic says my
magnetic heading was 356.
So, for safety purposes to avoid trying to occupy the same finite
airspace as one of my fellow flyers, should my altitude have been 6500 for
the mag heading -- or 7500 for the ground track? It is a tight corridor;
SUAs and cumulus-granitus don't allow for much zig-zagging to add ten
degrees to either side, which I've always considered unacceptable anyway.