I'd reiterate the comments made here. 120 liters to 80 liters, you've
just lost one-third of your floatation; or you've ballooned up to 310
pounds! That's huge. You'd need to be sailing in winds at least 10
higher than what you have been sailing to make it work, and that can be
very intimidating jump--the whole landscape of the water changes, and
the rig reacts to gusts and lulls in a much more dramatic way. Small
boards, by their very nature, are going to be much more sensitive to
weight distribution and mast-foot pressure, and you won't be able to
make the mistakes you can make on a 120L and get planing.
Too many sailors make the mistake of trying to jump down to a real
short board once they've got the basics down on an intermediate board.
Recently I met a someone who'd been sailing a 117L board and wanted to
jump down to 76 liter board. I managed to convince her to at least go
to about 84 liters, but I only felt OK about that because she had
sailed in 3.7 conditions before.
Dan, if you've bought this board and you're wedded to it, this would be
my advice, and this are just guesses since I'm nowhere near your size:
Don't go out unless you see other sailors of similar size using
similar-size gear out there. You'll need at least 30-35mph wind, and
probably a 4.7 or a 5.2. (Most 80 liter boards don't work too well with
anything bigger anyway.)
Practice your downwind sailing skills. You'll need to be willing to
head downwind to get onto a plane and into your footstraps. That's
gonna take some courage in the wind you'll be needing. And go to a spot
where the walk of shame won't involve scrambling too far.
For waterstarts, if you don't already, put only your back foot on the
board. Some people put both feet on the board to waterstart, leaving
their body extended way out from the sail and hindering their ability
to push the sail up into the wind, hence slowing their waterstart. Even
at the Gorge, you don't want to be stuck drifting around. Using only
one foot gives you more flexibility to extend up into the wind.