Chassis Flex vs Cracked Frame

[Jacob 76]

On my 97 F350 CCLB I'm in the process of building a front 4 link and subframe. My 4 link will be mounted off sub frame along with my rear traction bars. From the side it'll look somewhat like a monster truck. My front suspension I have figured out but I'm worried about my frame. Anybody who has an OBS knows the frames are extremely weak and flex ALOT. Just like truck frames, pickup frames are made to flex so I'm unsure if I will end up cracking my frame. The frame is boxed from the factory part way back, about to the drivers door, then is just C-channel the rest of the way back. My plan for my sub frame is to start it at the front body mount, about where the boxing ends, and go back to the front spring hanger on the rear springs. There will be 4 new cross members tying the sides together. One for the front driveshaft hoop, one for the ZF6 mount, one for the transfer case output hoop, and one for the carrier bearing driveshaft hoop. Now here is the actual question, the chassis gets "loaded" where the springs mount so if I end my sub frame just in front of the spring will it try and flex right there and crack between the sub frame and hanger or should I bring the sub frame back further and build the spring hanger into the sub frame itself? That way the frame isn't under hardly any load. Am I just being paranoid or is there a good chance it'll crack?

Kindof on a side note, does anybody have a program they could use to model the load on the frame? The same way the model the stress on 4 links.