I'm cotemplating dropping an 81 L28E into my 77.

What's difference? I plan on modifying/rebuilding the L28E (cam, etc.), and when I'm done, doing the switch.

Should I use the ECU from my existing 77, or should the 81 ECU be used?

Standard nissan enging naming conventions would suggest that the difference is that one is electronically fuel injected and the other isnt (ie its got carburetters). But I'm guessing. nissan might have not started adding an e to injected motors right from the get go.

And from the sounds of your post, you already have fuel injection. I'd used the ecu and wiring that came with the engine incase they are different.

If you go modifying the cam you might have to use an aftermarket tuneable computer to get the best results, as the standard ecu is tuned for a stock engine.