will a 727 bolt to a 318?

Magnum block is essentially the same as an LA block. Some minor internal differences, but otherwise identical. I think they were all internally balanced if I'm not mistaken, maybe someone else can chime in if I'm wrong. If you want overdrive, the 42rh is the way to go.
Here's a breakdown.

727 - 3 speed, non lock up converter. 727 came in big block and small block form, 904 small block only.
904 - 3 speed non lock up

42rh/e - 4 speed 904, lock up
46rh/e - 4 speed 727, lock up

All use the same bolt pattern (excepting the BB 727 of course), just a weighted or non weighted torque converter (converters are, of course, different between the 727 and 904), depending on if the engine was internally balanced or not.

So basically, if you wanted to run a 904 behind your magnum, just make sure there aren't any weights on the torque converter, and bolt it up.