John - I believe that is it, 60° = even alternating firing 1-4-2-5-3-6? I can go with the 2.5" as there was no reason not to do so except better clearances/easier to work with.
I did some research on exhaust systems some time ago, just generic exhausts and how they work. The reasoning for the X-Pipe is to assist with scavenging as you already know. I'm not sure if your long tubes will be as effective in scavenging without them.
Lets go over basics and see if the logic applies here:
The idea is that the X Pipe connects the two sides, we got that. The firing order alternates sides of the V (1, 4, 2, 5, 3, 6). So #1 fires, sends the pulse of gas out at a given velocity. After the gas pulse there is a vacuum created in the exhaust from the now lack of gas as it passes, this vacuum pulls the exhaust gasses out of the #4 cylinder by expanding the vacuum to the other side of the V as well through the X or H pipe. Then the pulse from #4 passes through and creates the vacuum from #2 on the opposide side via the X or H pipe.
So the reasoning here would be that your free flowing exhaust would still have scavenging. Logic would dictate that it would have to happen at a greater interval then, since each side of the V would now be separate. The pulses from #1 would have to create a vacuum and pull on exhaust from #2. The question is whether or not the timing between the intervals would be ideal for scavenging.
Also your exhaust is going to be loud as ****... When you get tired of your ears bleeding I'll take those Borlas off your hands for you : )