I'll tell you what; I just did an experiment on a 5.2 (318); a 3.9 is basically 3/4 of a 5.2. Same pistons, rods valves,everything (only 2 less of them) I had a set of stock Magnum V8 heads that I had spent $100 on a local machine shop "standard" V8 valve job. I brought em home and yanked em right back apart (on the kitchen table no less) got my dremel out (be sure to catch the bits on sale; you'll use a few and a clear full face shield, too) and I cleaned them up myself. I didn't even go as far as a gasket match. I cleaned up all the "casting fuzz" as well as cleaning the "short side" (bottoms) of all the ports and blended the seats into the bowl area to get rid of that "lip" where the backs of the seats hangs over the port. Thats it; my 1st try at porting/polishing heads. something I had wanted to do for years. I did end up nicking a few of the fresh cut seats, so I had to take em back to the machine shop to have my boo boos cleaned up (ground out) and another round thru the hot tank to flush out all the grindings. Next time, once they're out of the hot tank and verified not cracked I will do the porting before they do the seats the 1st time. (cost me ~$50 to have the seats done 2nd time) I put em on the shelf since the car that I plan on putting this engine into STILL is not out of the body shop; well, last week, I changed out the lifters chasing a tick in the motor that's in my 97 Ram fullsized 4WD (didn't solve; but verified my 1st thought; wrist pins) as long as I had to have the heads off to swap lifters, I stuck my fresh set on there. With just that amount of cleanup, I was blown away with the difference in the way that worn out dog tired old work truck ran! dead stock, 145K, and the difference in power and smoothness was very noticeable; no trip to the chassis dyno but my butt dyno definitely felt the difference! That's why I was so pissed to now be pulling the motor to put in a good used one as that old one, knock/tick be damned, has never run so well in the 4 years I've owned it! So YES, it is definitely worth it; maybe find a cracked head (dont need to be a 3.9; any head out of the scrap pile at the machine shop or junk yard would be good to practice on. Im sure they have a good selection of 318/360 Magnum heads to pick from.) Then go to work! And unless that last 0.001 off the track time is your goal, you can do a more than sufficient job at home yourself; put that $$$ you want to pay for CNC somewhere else (in your stereo or a set of rims maybe) or if nothing else; send it my way! But if you're not sure how much you can grind off, err on the side of leaving a little extra behind. I only put ~200 mi on the truck before I pulled it out to swap; but I did learn quite a bit from it; as I was putting the intake back on I was looking at it (stock kegger) and noticed at a minimum how many sharp corners that could be cleaned up on that, as well. That don't even start to consider the cutting down of the runners within like so many guys here can probably walk ya thru.. But being that my current goal with my truck, is a motor that runs reasonably well, with no abnormal noises, that I can put the "for Sale" signs back in and have a reasonably good shot at selling it for a decent buck (once-a-month disability+11MPG don't mix any more) so that means I don't have to give up these ported heads either; I may pull 'em back apart and dig a little more on them before I put em on that fresh block that I have on another stand. (I will eventually get that car back home from the body shop, done or not; and when I do it'll need a motor) I'm replacing a Slant Six with a 98 318 Magnum, in that project.
