wow, ok, yeah thank you all for responding! I am a little bit overwhelmed with all of it haha, but Id rather have too much info than not enough.

anyway to give some responses to the above, I have a 10sp cassette lying around in the garage, I was going to buy a new wheelset to use with that cassette and put on the bike, assuming that it will fit the bike (I still haven't measured yet). And not even sure what exactly to measure, the space between the QR brackets? or the axle length or is that the same thing. And what type of 'wiggle room' would I have on a steel frame bike? any? is that 4mm difference someone mentioned too much or ok?

currently the DT shifters and the derailleur are shimano 600 series. but it's weird because the shifters are friction, and the derailleur is labeled as SIS, but they are a set? But that is what I have planned to do. I did notice the 6sp on there now is a lot wider spaced between the gears than what a normal 8/9/10sp is, so, MAYBE I would be able to use the derailleur that is on there now also. Not sure.
thanks again for all the tips and suggestions! I like the shimano shifters mentioned above by a couple of you. If the 600 shifters don't have enough range I will likely look at the dura ace, although they are a little high.
I'll try to get in the attic to take some measurements this week.
EDIT: oh yeah, sorry it's a late 70s Trek 510