As to radar, with the USA, I always build a few: the Cape Cod province, whatever it's named (I always forget), Honolulu, Colon, Puerto Rico, Seattle, San Frnacisco, and San Diego. Yes, I know I don't need most of them, but I do it for historical purposes. I won't do the massive upgrades to Guam/Wake/Midway/Johnston Island that usually tempt me, since they pretty much win the game right there.
Folks new to the USA should try it, though: max out Infra, airfields, and ports, and once radar is online in '39, slow-build (one at a time) radars on Wake and Guam to level 10. Research provincial AA to current year, and have level 5-10 AA on the islands, too. Put in level 5 coastal forts, too, and man with a few divisions, and watch Japan beat its head against the wall. In vanilla, I usually go with GAR/GAR/AA/ART divisions in the Pacific, anyway, for defense. Two-three of those will see Japanese CAG lose ORG when coupled with provincial AA, and with INT and/or MR based locally with intercept missions, it's too easy.
Add in a few 2xNAV air wings patrolling the coastal waters, and Japan will lose its main fleet by mid-'42, if you use your fleet properly.