EFI ECM: which would you choose and why?

I suspect that 67Dart273 is talking about the ignition system. The Ford EDIS is very popular with engine controls hobbyists and Megasquirt recommends it as the easiest way to distributor-less ignition. I got the parts for future playing and already put a 36-1 wheel and pickup on my 273.

However, the OP's question concerns fuel control. I would prefer Megasquirt because I am an engineer and programmer and can dig into the electronic board and even the source code if I must, which isn't possible with a factory setup like GM. The factory box approach is probably cheaper, but will always be a mysterious black box and you will have to rely on after-market companies for chips (I think).

I sort of gave up on Megasquirt because the design is a bit dated. Especially, distributorless ignition requires many clunky add-ons. Even MPI gets clunky for a V-8 since the original design was for throttle bodies (2 injectors). Also it isn't cheap, though I keep trolling ebay hoping to snag one at my price point.

Most stay away from the Mopar engine controllers because there is less tuning support. However, the 5.9L box should be a direct match to your basic 360 engine, other than your cam changes and such. I got one of those boxes and harnesses for $15 off ebay, just to maybe pop it open and see what chips they use, plus get the connectors. I may even try running my 273 engine with it later.

I wonder what you will do to get a MPI intake manifold. Are there aftermarket ones that will fit the 360 head bolt angles? Are you switching to Magnum heads? I bought a Magnum "beer jug" MPI intake w/ parts for my future playing. I may be able to mount it on my 273 heads since my bolts are much closer to vertical (see my post). If not, maybe I'll get Magnum aluminum heads (I think they make them) to get the right bolt holes and shed ~50 lbs, plus I suspect the valve covers seal much better.

Finally, don't overlook the effort of making a wire harness. Many people treat that as an after-thought. At work, I often deal with making custom cables, and is neither fast nor inexpensive. Just look at the prices Painless Wiring charges. Whenever you can leverage a factory harness, do that. Otherwise, your EFI will never get running because the cabling turned out to be too much. I suspect most Megasquirt projects end up that way because they have you both build the circuit board and fabricate a custom harness.