The reason I don't want to use brass is space and weight. I know that sounds insane, but it's a very small amount of space and the right combo of brass fittings is quite large and I don't like that pointless space and weight, and I hate having my engine look like I paid a plumber to fix it.

I bought the correct array of fittings in aluminum to make it at least look like I tried, but it's still a physically huge intersection. There's no reason you should not be able to use plastic here, zero. I have a Gates 5/8" tee on the other side connecting the water pump to the "head tube" to the coolant reservoir, just inches from the exhaust manifold and it's been fine for a very, long time. This problem is purely poor Dorman quality... it's actually pretty sad because the crap Dorman part cost more than the good Gates part.
But, it's okay, it's on the past. I paid good money for the spacer and it's going to get used!
It was kind of an annoying fit - I could not get it placed without removing the valve cover - the Felpro Permadry gasket got in the way. Hopefully this activity does not result in a leak.
I bought some Permatex "Permashield," which I've read is a clone of Hylomar Universal Blue with a cheaper solvent. It's $9 for vs $30 for 2oz, so I'll live with it! I used a foam brush to spread it all over the phenolic spacer, then "glued" one intake manifold gasket to each side. I hacked off the heads of a couple long M8 bolts to use as dowels to get everything lined up and install was pretty darned painless. The EGR tube required minor adjustment to fit properly, not a big deal.
My instrument cluster gauge hasn't worked in probably a decade (bad sensor, not gauge) - long ago I installed a VDO temp gauge with the sender in the DG/MC2 aluminum thermostat housing, so that was already taken care of. I still had to relocate the ECM CTS, so I used a Four Seasons 84540:
I don't know what it's for. Rockauto says it's an AC fitting that fits no cars.

I placed it in the feed to the heater valve/core - it's where the CTS goes on dual-plug Mustangs, hopefully whatever slight differences in temperature it sees don't upset anything. I can't imagine +/- 10 degrees or so would... not on EEC-IV anyway. A Gates 28467 (5/8" L hose) got hacked up to go between the tube on the head, this fitting, and the heater control valve. A second 28467 plus a Gates 18701 (5/8" tight U hose) got hacked up to return the valve to the tubes. Looks very factory, I'll post a picture of it later.
One less hose, maybe a cooler intake, and hopefully no more coolant all over the place!