I BELIEVE THE CROSS TUBE IS TO EQUALIZE PRESSURE AND GIVES A LITTLE MORE TORQUE.

stock brake drums , but have the pin drive adapter for the wire knock off wheels bolted to them. makes them look like Buick finned drums at a glance.

I COULDN'T SAY FOR SURE, BUT 25% SEEMS A LITTLE ON THE HIGH SIDE. IT WOULD BE DIFFERENT FOR DIFFERENT COMBINATIONS OF PARTS. SUCH AS CAMS, HEADERS, INTAKES, CARBS,ECT.

Thanks kindly for all the comments.  I think I may try the exhaust as shown but eliminate the cross over.

thats a slick setup, i imagine not a noisy as my headers with cherrybombs under my ass and 2 feet of pipe from them!!LOL I like how the mufflers are mounted in the back, i might try this if I actually put on a full exhaust

Which is your supplier for exhaust pipes?  Do 2.5 inch pipes go over the rear axle without any clearance problems in the vicinity?  Or do you run your pipes out in front of the rear wheels?

...we had a custom system installed on ours by our local Midas shop. They said because of the awkwardness of the X-frame setup they couldn't install the crossover pipe, which I wanted (crossover pipe is supposed to equalize the exhaust pulses between each cylinder bank and give a bit more power and smoothness - I agre that that claim of 25% is way too high though. I think 5-10% is more in order..) Then our mechanic told us that GM intake manifolds had a crossover built into them unlike, say, Ford smallblocks. I have never been able to verify that anywhere else so really don't know. I also don't know if a crossover in the intake mnfld. fulfills the same function as a crossover "pipe". Anyway, we have a non-crossover system, for now anyway, with Cherry Bombs directly underneath the pass. compartment and I can't say the noise at cruising speed is unbearable. But you know you have Cherry Bombs! Which is what we wanted anyway! We're basically happy with our 2" all-the-way-back system. We have the pipes exiting at the sides directly behind the rear wheel wells, with foot-long chrome 3" extensions that don't extend past the body panels ( too greaseball...), which looks very clean and has also drawn lots of compliments.. 

That's good to know.  I was leaning towards 3" pipes as well. I guess you had no problems placing the cross over.  ,....'66 was already uni-body,. right?  I understand that '64 was the last year for full X frame  ??