Hello Vanagon Types

I just returned from a camping trip to the Sawtooths and I have a question for the group about my van's performance.  This is a '85 Westy with a GW 2.3, and recently installed GW Hd rear drums. It is my first trip away from home with the new drums. It is running well with one exception, which is what my question is about. When traveling north from here I have to go over several passes, including one at 7,000'.  In the past, I would downshift to third on these passes but not more.  On this trip I had to go to 2nd near the top, and the van felt like it had less power when climbing.  Less power at that altitude is normal, but this is less than my prior experience, and I drive these passes several times each year.

My question is, could the HD drums have a negative impact on performance.  The van does not have less power, but could it take slightly more power with HD drums so it feels like it has less power?  I do not notice it at home and or level ground, and it can easily do 70 or more on a flat highway, but slows more quickly than before when I am climbing on a grade.

Does that make sense, or should I look elsewhere for the cause?

Thanks

Dave B
Boise