Those captures look quite good already, and it is clear that averaging the captures has had positive results, as for the levels I'm no expert but I'd say you need to reduce the brightness and increase contrast on the capture card settings to try and use the full range of the capture card, this will give much better results than adjusting afterwards with Avisynth.

When setting the capture settings the letterbox bars should be your best black reference so reduce the brightness until they are just above the minimum, then adjust contrast until the whites are just below the maximum, if the subtitles are white they might be a good reference for white levels, otherwise you'll have to search for some scenes that have pure white in them.

If possible (and I'm no laserdisc expert so it might not be) find a frame with a pure white area and pause on that frame, then adjust the capture settings while running the waveform filter in Vdub, I think this works with my capture card but yours may be different. I'm sure Moth3r will correct me if I've got anything really wrong.