Rnunn,
Well...
I tried both ways...
I read some posts on this issue where some were concerned that if you don't fit the tubes with them bolted on tight to the head, you might struggle with leaks at the head (because you will be making brackets, etc, w/o the flanges perfectly square to the head). So during my initial fitment, I bolted the tubes on to the head tight, and then had to become quite clever in getting the side pipe to slip on: I broke a paint stick in half and put one b/w two tubes horizontally, then the other between two tubes vertically. Then, because the tips of the tubes weren't perfectly aligned with each other, I used a ratchet tie down around the 4 tubes and tightened it so they formed a tight square pattern with the sticks in between. So if you're looking at the pipes from the hole in your fender, you'd see the tips of 4 tubes separated by the thickness of a paint stick, paint sticks forming a cross, nice and square - then the side pipe slipped on pretty easily with the forced alignment. Once I was sure the side pipes were on far enough, then I welded my tabs and made my rear bracket with the pipes parallel to the car, etc. Did I mention this was about the hardest part of my whole build? Seriously! Use some WD-40, too.
After disassembly and ceramic coating, then I slipped the tubes on the side pipe loose before bolting to the head. I figured since my tabs and stuff were all force fit previously with the tubes square to the head, they'd go back to the same spot. Which they pretty much did. I also wire brushed the inside tips of the side pipes to get rid of a little ceramic coating as well as sprayed some WD-40 on the tube to ease the slip. And I don't think they leak but haven't driven it enough yet to be sure...
Disclaimer - there are probably better ways of doing this job...I thought it sucked! Especially since I left the inner fenders in place - wouldn't want to make it too easy!
But, assuming the holes in your car are big enough, you should be able to attach the pipes either way - with or without the tubes already bolted to the head.
I honestly don't like the design at all. If I had it to do over again, I'd attach them like SPF does their Coupe pipes, (not using these tabs). The tabs merely hold them in place. The SPF Coupe style of doing it actually pulls them together tight as well as holding them in place - and they are slip fit...anyone have a pic?