You don't have to change the rake on the frame. Your swing arm will have to be angled downward to get the ground clearance you need anyway. This will pull a few degrees out of the rake. Next, when you have your tripples built move the fork tube holes more forward of the steering stem hole. This will lessen the "trail" part of the steering geometry to take the "heavy " feel out of it. There is a lot more to steering geometry than rake and too many people get stuck on it. Dirt bikes will have about 4-6 inches of trail where as a trike needs 2-3 inches. The trail number is the one that makes the most difference in the way the trike is going to handle. Cutting the frame and changing the rake angle is one way to do it but not the only way.
Also, if your modeling it after a 250r frame keep in mind that a 250r has a significant amount of extra rake built into the tripples themselves so you might not be as far off as you think.
YAMAHA 450 HYBRID
85 350X- RED
85 350x -BLACK
86 350x-WHITE (with Goki)
85 250r
83 atc 70
84 atc 70
84 atc 110
09 yfz 450
2006 Arctic Cat Prowler
RZR XP 900