Doug's exhaust help '66 Dart
have you looked into the tti? they are a little larger tube then the dougs. and cheaper it looks also..
don't know how much of a gain you'll see but i do know the 273 manifolds are choking the hell out of it now.
dougs- Primary Tube Diameter (in) 1 5/8 in. Collector Diameter 2.500 in.
Ceramic Coated $879.95
------------------------------------------------------------------------------------------------------------
tti- 1-5/8" O.D. primary tubes / steps up to 1-3/4" O.D. secondary tubes / merged into 3" collectors...
Ceramic Coated with a Thermal Barrier $776
http://www.ttiexhaust.com/Header-TTI636A/63-66Abodyheaders.htm
.