On mocking up the rearend, you should first figure out what angle your driveline is at. My engine sits ata 5 degree angle to the frame. With that said, most vehicles have a 2-3 degree rake, so technically the engine/trans sits at 2-3 degree angle to the ground. Most rearends are setup with a negative1 to 2 degree to help compensate for axle wrap. I hope that helps.
I see you bought a CPP Ifs, I purchased their Economy crossmember last year. Anyhow, have you established a ride height? Off hand, it looks like my crossmember is going to lower the truck around 4”. I’m planning on adding some material because I don’t want it that low.
