There are no digital copies of the '77 stereo mix on laserdisc. Belbucus' analogue capture was made with very high quality equipment, so it's probably the best it's ever going to sound short of an official transfer from the master tape.

As for conversion to 48 khz, if this is not done well, much of the advantage of using digital audio will be negated. While resampling can never retain the same quality level as the original file, the degree of degradation can be vastly reduced with a high quality algorithm. The iZotope processor (which I have) may well be one of the best ever made. Very few others can come close—even the ProTools resampler isn't quite as good as that.

If new tracks are going to replace the captures from before, then they deserve to be heard at their best. If need be, I may be able to help out with this. ;)