Modify a 1629 magic eye tube circuit to a 6E5S tube?
I know the first step to this is to reduce the filament voltage to 6.3V for the 6E5S. But something that bugs me is, can a simple resistor voltage drop get it done? 


No, it can't. Note that the filament supply is 12.6 V @ 0.15 A for the 1629, while it's 6.3 V @ 0.30 A for the 6E5S &mdash; half the voltage, but twice the current. The power transformer has a 6.3V tap on the filament winding; all you need to do is connect the filament between X and Y on the transformer (instead of between Y and ground).

I can't see any details on the radiomuseum site (not a member), but I'd be willing to bet that most of the other parameters of the two tubes are very similar, so once you get the filament worked out, you should be good to go.