Per HerHusband:
I don't know how big your antenna is, how tall your mast is, or if it
runs all the way to the ground. I have a five foot mast mounted at the
top of our gable end. It's simply lagged into the 5/8" plywood sheathing
with two brackets, haven't had any issues in 10+ years.

It's waaaaaaaay bigger that I expected - as in 32# and 7 x 11'.....

Accordingly, the mast will butt up on a cinder block on the ground.

Main thing, I want to be able to re-aim the antenna without climbing a
ladder... but having it rest on the ground would also seem to alleviate
a lot of stress on the W-brackets holding it to the side of the shed.

I think I will go with the 2x4 just for the spacing - to bring the
bracket out to match to top one on the eve.

I like the washers... definitely going to do that.
--
Pete Cresswell