Call Garhauer, they're a class act. FWIW, I notice you've got Harken
blocks now so you could just look the blocks up at Harken's site and
see what the working load is. You might also try Harken's rule of
thumb formula for mainsail loading:

ML = E^2 * P^2 * 0.00431 * V^2 / (sqrt(P^2+E^2)) * (E-X)

where
ML = sheet load in lbs
E = foot length of main in feet
P = luff length of main in feet
V = wind speed in knots
X = distance from aft end of boom to mainsheet attachment in feet.

I think the mid-range 2k SWL blocks will likely work for you.

-- Tom.