Those fittings - here's the catalog page at
mcmaster You want part 53415K212 - and you have to specify in the "comments" when you order that you want BSPT. There doesn't seem to be a 1/4BSPT to 5/8 or 9/16 barb, but the 1/2 should be fine with your 1/2ID tubing - just might need a cable tie or something to secure the tubing.
If you're considering a larger than 120mm, there's a guy over on the
PC Perspective Liquid Cooling forum who,s been selling heatercore/shroud/fan combos that he builds. Comments from buyers have all been favorable. He goes by the handle
WA2.
Then there are the "chevette core" sized heater cores (fins are ~6"x6" - top and bottom tanks add about an inch. There's a pre-made shroud especially for these cores made by
CoolingWorks that make mounting a heater core really
easy.
Guess that's it. I've never used PolarFLO, so I can't comment there (the pictures on their site sure are pretty, though...)
Bob