...have any of you making these suggestions actually had or have a 4.0L? (I've had several, and done quite a bit to them.)
First of all, we need to know what your plans are. The engine, in stock form (except for valve springs), easily supports 400 hp/tq without any internal modifications (I'm running a stock engine with 400+tq with my turbo, and have been for 2 years). You won't exceed this power level with any f/i other than a turbocharger, so if you intend to put a X-charger, vortech, or procharger, a stock engine will hold up just fine. If your goals are to go more than 400, then you either should so some kind of forged built, or better yet, opt for those "extra 2 cylinders" and put in a v8.
A better option for you would be to find another good replacement engine and drop the whole thing in, and sell the engine you have. You'll save a lot of money in labor that way.
If you do want to rebuild it, I would go ahead and put in Nissan 909 valve springs while it is apart; that'll take care of typical valve float at higher rpms.
If you intend to rebuild it yourself, know what you are getting into with the cams. They are a pita to get properly aligned.
Porting and polishing the heads will give you some nice gains, and if you were to do a rebulld or forged build, head work is something you should do.
Even if you do a forged build, the block then becomes the weak point. People have broken the blocks in the mid-to-upper 400's.
If you want to get more info on some nice 4.0 builds and issues and tricks and stuff, check out some of the build threads on mustangcollective.com.
Mike -- 2005 "The Beast" (Formerly a V6, now a forged/stroked 5.0L V8 sportn' a Kenne Bell 2.6L @18psi, and many mods
2006 4.0L w/Powerhouse turbo and TR-6060 :kooky: