Actually the disc brake kits will fit '37-48 spindles. '32-36 have the smaller bolt spacing with smaller 3/8" holes. '36's have the same snout as the later ones, and will accept all '37 up drums, the '32-35 snouts and need a spacer to correctly align the drum on the shoes. You can fill the 1/2" holes in the later backing plates, and redrill them with the smaller pattern with 3/8" holes.

So, drum brakes, '28-48 spindles will all work, with the '28 to '34 spindles presenting a few steering arm issues(no cross steering), etc. If you want the current crop of available disc brake kits to fit '37-up.

Round Backs bring more purely on aesthetics. Square backs work better with the '48-56 F1/F100 brakes, they clear the wheel cylinders, you have to do some grinding on the '37-41 round backs. The $280 number is incredible - but they aren't making this stuff any more, and the traditional hot rodding scene is growing very fast. The number of guys in their 30's building 100% Ford cars with no aftermarket or repro parts is incredible.

I would prefer forged over investment cast for my car. SB & Magnum have sold a ton of cast spindles over the years, so you are right, they can't be that bad. For new spindles I actually prefer forged ones from Chassis Engineering. I think their spindles and I-beam axles are the best repros.