New guy in town!

Hey everyone, just joined. Name's Jon, from Tri-City Michigan area. Heres a picture of my '74 Swinger the day I bought it about a year ago. Its bone stock, all original 318 A904. So far I've got the trans totally re-done with a Trans-Go stage II kit and 2,200-2,500 stall converter. Got the engine out and about ready to drop the heads off to get them milled. Installing a Comp 268 cam and lifters, MP springs, Edelbrock performer with 600 Edelbrock 4bbl, Headders, 2.5 duel pipes and updated orange box ignition. I have a few questions: Which type of headders fit the best (I do have power stearing but not power brakes) and also, what is the max I can mill my heads to bump up the compression? I'm stick with stock rockers but I may shim the rocker posts if needed. Any tips? Thanks!!!