Great lookin' little stepper there!
Start with the intake and exhaust, and ignition. Offy is streetable, and the Holley 390 should be great. Check out E-Bay for a set of headers, otherwise keep an eye on classifieds if you don't mind used parts - $400 for a set of long tube truck headers is outrageous, in my opinion. SO yes, the Summit headers will still be a very big improvement over the stock exhaust manifold. And remember to run water heat to your intake for better fuel atomization.
After that, a cam is the next step. You should be able to pick up one for less that 200, I'd think. Check around, a couple cam companies have been goping under, but last I checked, I thought Comp and Crane carried I6 cams, however, I may be mistaken. Check Schneider's cam's as well.
And as far as heads, a 194 head will give you almost a full point in compression, which is always helpful. You may be better off finding a good quality head, and start putting money into it after the other bolt ons are done to the engine. Start with valves, lumps, porting and polishing....etc.
But, that's what I'd do. You're off to a great start in the right direction!
-Sam.