You will be chopping...
I think the lightest DH heads in Guideline 3D are the 8/9 series at 540 grains, so you would be looking at a 90 grain reduction, somewhere in the 5 - 6 foot area of reduction. I scanned the Guideline catalogue and the next step down from the DH 8/9 series is a SH series at 24 grams, 400+ grains. Maybe you need a different rod! I use the F/H/I and the F/I/S2 most, with the F/S2/S4 coming in third. The rivers I fish here on Southern Vancouver Island are the spate type, freestone bottoms, lots of broken water, short deep slots and pockets, not much in the way of long broad runs like the rivers up north in B.C. So when I got the heads, I was more interested in using their full capability than fitting them to an existing rod. My 3Ds came from Ireland at £80.00 each, ouch, so buying a rod to fit the heads was the most logical step, rather than cutting the heads to suit one of my rods. I got a used European – made rod perfect for the heads for $350.00. What a great combination!