...I've decided that a better option is to get a 5.8 shortblock from JDM and have the heads done at MHS...
For the money you will have invested, I personally would stick with only one shop to supply your long block assembly. JDM is my shop of choice for quality workmanship, and I am fortunate that they are within driving distance from my home. I am also impressed by MHS's level of knowledge and attention to detail, and I think that you would be pleased with the results from either shop. I would just be thinking about my recourse if something bad happened once the new engine was fired up.
Would you be doing your own final assembly of the engine, or would a third company be involved?
I would like the weight savings of the aluminum block if road coursing my car often, but in my street application, I would likely just bore out my iron block to a 5.8 when a rebuild is necessary, if the block is reusable.
Finally, if your engine isn't tired and worn out (burning oil), I would have no issue with installing your purchased "go fast" parts on your car and letting it eat!