Nothing wrong with basing the build around the GT40P heads. There's not much port work to be done on them since there's not much material available for it and the heads are very well optimized from the factory. A freshening up, cleaning up the bowls and castings would really be all that's necessary IMHO. You might be able to use some pedestal mount roller rockers which would be a decent upgrade for the valvetrain - I think Crane Cams use to make a set of their diecast Energizer series roller rockers and I ran those for many years before selling them to another individual.
You could send the lower intake out to TMoss for porting which would land you some solid gains on the induction and may better support the upgraded camshaft you're planning. As for exhaust, the engine is far more sensitive to intake restriction than it is exhaust, but upgrading to headers will certainly free up some power.
When I recently built a new engine for my Mountaineer, I used an old set of World Products cast iron heads - I did this intentionally to avoid dissimilar metallurgy on the interface between the heads and the block because I tend to use the vehicle for long distance trips occasionally into remote locations. Also, I had the heads on hand and they were fully ported. I am not disappointed in the performance at all and I could care less about weight savings in a vehicle that already exceeds 4000 lbs.
I'm guessing that it would have cost around $500 to have the heads reworked by a local machine shop. I didn't actually ask, but that's the number I'm assigning to it. Another $200 for upgraded springs. These heads come with all of that already done, so the way I see it is I have about $300 into the porting work that they do. I considered attempting a DIY clean and port, but it was worth it to me to pay someone else to do the work. I'd have more dollars invested in the time to do it and wouldn't have gotten as good of results.
For the rockers, I'll be using stock rockers for now. I'd like rollers, but honestly not a lot to be gained for my use case. I've got a set of cheap procomp (IIRC) 1.7 roller rockers installed in the current engine with the explorer cam. Since I'm cam swapping I don't think I want to run the 1.7s and I'm not going to install those cheap rockers in this new engine. If I don't find something used in my price range before building, they will be easy enough to change down the road. Maybe if I can get a 1.6/1.7 set? 1.6 for intake and 1.7 for exhaust to get more out of the smaller valves in these heads. I'd need to do some more research on that idea, and should probably see how it runs with the cam first.
TMOSS... I thought about that. Biggest reason I didn't go that route is how he works. It's a turn-around service, you send your manifold in for porting. Also difficult to find information on how to contact him. It's out there but hard to find. Also he doesn't do the heads. Not discounting TMOSS's work, I decided to go another direction. I ended up going with a competitor that sells outright, does heads, and is easy to contact through Facebook. I ordered ported upper & lower intake and EGR elbow to go with the heads and cam. When done I'll probably post a set of heads and intake for sale somewhere and recouperate some of the cost.
Already got a set of OBX headers installed. They fit fine, but after I finish getting this engine together and recouperate some of the bank account, I'll probably order a set of TMs for future use. I wasn't willing to spend that much and deal with the BS (wait time, lack of contact, etc) before, and wasn't going to do the engine swap with stock manifolds. Now that I don't "need" them and people are actually recieving them, I'm slightly more willing to deal with the BS.
Rest of my exhaust is kind of cobbled together at the moment. Got the Explorer stock down pipes back through the second O2 sensors. From there it's dual 2.5" into single Magnaflow muffler with single exit, not sure on exit size. I wanted to do dual in/out muffler and run duals to the rear, but since I was planning to lower the truck the shop was against it. I'd have taken it elsewhere, but at the time I needed an exhaust on the truck pronto. Looking at the truck now I'm not happy with the fitment of the down pipes and primary cats. I'm going to get the initial round of lowering done soon, then take it to a different shop to get a full exhaust run. Probably 2.5" all the way back with highflow cats and a crossover. Muffler selection is probably going to hurt flow a bit though, I really like the sound of the generic turbo mufflers over most anything else.
Dissimilar metals are part of the reason I wanted to go with iron as well, probably for the similar reasons. In my line of work galvanic corrosion is a big problem, but I'm not too worried about that. Different rates of thermal expansion is where my concern lies. Aluminum heads on an iron block is theoretically more prone to head gasket issues. My main plans for the truck is fun street driving so it would probably never be an issue, but the truck may be worked kind of heavy on ocassion, it is a truck afterall. I wanted the added percieved reliability of iron heads on an iron block. Like you I wasn't concerned about the weight savings, the few extra lbs aren't going to be an issue in this application.
