wow ya your numbers are VERY close to what I have found. From my calculations the headers will be 36.9 inch primaries, and the I.D. should be as close to 1.55 in as possible which would be 16 gauge 1 3/4" pipe.
Yes this is going to be very hard to fit under the car since the current headers are REALLY small, but I am planning on just pushing the exhaust back as I am removing the precats anyways.
I am also making the exhaust in such a way that I can add in an H or X pipe later, I want to see how the car will run and sound without it for now.
Lastly, I was thinking about making the exhaust 2" but A) I found it easier to find parts in 2.25", B) even for a venturi collector system it is suggested not to make the venturi less than 1/2" more than the primary pipe size, which is already 2.25", so 2" would be like putting a really small venturi on it, but I'll look into this deeper. C) Like you said, most of the rest of my car may still be stock now, but not for long, I wanted to leave a little (but not too much) leeway for MORE POWER
I am building the system in CAD to get some really close tolerances, and I'll also be testing the model in Flowworks, so once I've got it designed I'll post a picture of what they're going to look like.