The heads and tailcaps on all SF E-series lights are compatible and interchangeable. The only thing you need to keep in mind is that lamps, LED modules, etc. tend not to be compatible with both the one- and two-cell formats. If you're looking to put the tail and head of an incandescent E2D onto your E1B's body, you'll have no trouble fitting the parts together, but you'll want to get an E1E lamp (MN01, I think) as well, since the E2D's lamp (MN02?) won't run on the 3V of a single cell. And although this setup would work just fine, the lamp for the E1E only puts out about 15 lumens, while the lamp for the E2D puts out about 60.
The same principle applies to LED heads as well. To my knowledge, their one-cell heads are not designed to handle the voltage of two cells (so your E1B head couldn't go on your E2D/E2DL body). And SF's two-cell LED heads (such as that on the E2DL) don't light up on a single primary cell. So if your intention was to stick with LED on this E1B lego, the head from an E2DL wouldn't cut it.
Then again, the above really only applies to stock parts from SureFire, and using only primary CR123 batteries. You could certainly look into rechargeable options, which might allow you to run the E2DL head on the E1B body. You could also look into something like a black annodized KL4 with crenelations (for related info, look up the "TW4"). Or you could go fully custom, and send an E2D head to someone for modding to an LED of your choice, as well as your desired range of voltage input (i.e., what battery setup you intend to use). Oh, and if you do want incandescant, I believe there are a few aftermarket lamp assemblies that put out more than SF's 15 lumens.
Anyway, there are many options, and the short answer to your question is "yes". Do look up the TW4, though - I'm fairly certain a number of people out there have set up lights very close to what you're looking for.
