Exhausting Decision?

[OLDFLM72]

Here's the current delima I'm having with the exhaust for my car...
Long story short, I was given one of the first prototype crossflow mufflers, but it's not straight through (has an open 6" chamber in the center) and might actually create a restriction in exhaust flow. Add to that, the Pypes mandrel bent pipes won't work with it so I'd have to have a local shop compression bend 2.5" to make the muffler work.

I'll be about $200 under ($400 total) what the Pypes Crossflow system would cost if I bought it from them and have to admit I like the idea of having a 1 of 2 prototype muffler. I just want to make sure I spend my money wisely and that I won't wish I'd have just spent the extra $200 later on... and put a "cork" on the back of my exhaust system.

Do I need any backpressure or should I go with a high flowing muffler for a 425hp Pontiac 406? (In my 72 Formula) Goelich? UltraFlow? MagnaFlow?

Should I just spend the $600 on the Pypes Crossflow?
Or spend $500 on a dual muffler system?
I guess that's the real question.
Would appreciate your thoughts...
V/R,
Ty