Stock exhaust manifolds vs headers

Depending upon your budget... I would prioritize head work, around the valves, or larger valves before spending $$ on headers... for two reasons.. One it allows for headers later if you want more performance, AND the head work is already done.. If you spend the money on headers, with small valves and no bowl work, then the headers won't add as much..

Most of the speed and performance parts for a slant six are limited by head flow... Make sure that the head gets the attention it needs, and then throw the intake and exhaust parts on it after a year or two if more speed is needed. Well the factory 2bbl intake made a difference!

Now I am in the process of building my own Dutra Duals, and converting a stock cast iron intake manifold to a spread bore design.. I have not done any head work.. BUT I mostly have labor invested, and a good running engine.. If I was doing a rebuild I would be focused upon head work as much as the short block.

A couple of fun articles on the slant six.. be very aware that there is a cheaper way ( and better i would argue) to do the ignition, intake manifold, and Dutra Duals. Machine work is going to be the same on all full rebuilds.. AND understand that the articles are largely an advertisement for the 'speed parts'.... still it's fun reading and shows what others have done with lots of pictures...

A three part series: https://www.motortrend.com/how-to/making-mopar-slant-six-sense-part-3/
And a bowl hog cutter was used under the stock valves here, as well as good head work: https://www.motortrend.com/how-to/add-77-horsepower-mopar-slant-six-basic-bolt-ons/