You would lengthen them, not shorten. The difference is about 3/8"-1/2". Or, just measure the fender opening to the edge of the wheel lip, tire or hub center and determine how far back you wanna relocate it.
If you have a really good eyeball, you can set the new arms by just measuring them against the stock ones. If you wanna ensure repeatable accuracy, a cheap way is to use a stock control arm as a guide to drill two holes in a straight piece of 2x4. Get a drill bit that fits snugly through the bushing eye to center it as accurately as possible and hold the bit/drill perpendicular to drill straight. Drill through the board and tap the bolts through the board with a hammer. Mark the side of the board the bolts extend from (opposite the bolt head/entrance side). A line across the length will do. This will be the "top/surface". The bolts will be spaced the original distance of the control arms. Cut the board in the center. Place the short edge of the 2x4 against the short edge of another 2x4 or 4' level or something straight and rigid. Spread the cut halves apart so that you have a 3/8" to 1/2" gap between them. The point of the drawn line is in case you get one end of the cut 2x4 turned around, you'd have a reference to properly orient them. Clamp the halves in place on the straightedge. You now have a jig to adjust the new control arms to whatever amount you wanna lengthen them.
Adjust the uppers by the same amount and install. Then, adjust them as necessary to dial in the pinion angle as it has likely changed some with the drop parts.