Reply to "twin turbos"

Wes, O-ring�ing the head is due to the maximum pressure at RPM, at low pressure you don�t need it. The compression that turbo engines is about 8.0 to 9.0 and allow the turbos to do the rest. If you are making 500 HP now you probably are at 10.x so either lower the compression ratio, and pressurize to 650 HP or the route you originally considered keeping the turbo pressure to a minimum. Be careful to add PSI to an already high compression engine, I think 7 psi maximum, this arrangement should minimize turbo lag.
As far as looks and cost, you can still use the 180 headers with modified collectors, but the long runners on the intake side may effect driveability.