tubing question for frame..87 gmc

[boomer]

I put a section of 2by4 box tubing in my passenger frame rail. I clamped it to the top and will wack off the bottoms.I ran it about an 1/8 of an inch past the front cab mounts and ran it back 25" inches. That piece was straight. However the rail kicks up as it heads towards the back of the frame.So the next piece i join to it will not butt square to it when i run it along with it. Do i just tuck it in there the best i can to the next piece i have in there and join them together with some flat bar or fish plate and then continue on welding the top of it.

I seen www.lowerred.com and he said theres about a 3 foot section that prevents you from laying rocker. So i am assuming the 25 inches i have from the cab mount back isnt eneough yet and ill have to run another 12" inches or so where it kicks up.

This is on an 87 gmc "on air"