The top angle and flange looks right. It should be 3" dia. The problem you will have is that the length of the tube between the bends will be the wrong length unless you are very lucky. I am using the stock 4bt down pipe from the P30 on my conversion. I did have to cut the pipe between the bends and shorten for my truck. To get my length right I cut the tube and had the bottom bumped for a slip fit. I slid the 2 pieces together on the truck like I wanted then tacked it. Then I removed it from the truck for the finish weld. If you are not using the H1C turbo the flanging may not be right.
Thanks for the help guys. I figured I'd have to mod the lenght between bends and stuff. As long as it fit's on the turbo, I can work with the rest. This is 6BT down pipe, I believe. I do have the H1C turbo.