Can anyone tell me what I would need to do to insulate my 91’ LS hatch? 
I guess it depends on the type of insulation you want to use.
I remember someone using some type of liquid rubber stuff for noise insulation, but it probably adds too much weight if you want to do the whole car.
I used aircraft insulation which helps with heat and noise. It comes in a roll and I simply cut them to fit with a exacto knife and stuck them into place behind the interior panels with metal tape. I think I used 3 rolls of the stuff.
I did everything doors, floor, roof, and trunk, all EXCEPT the firewall, and it made a decent improvement in noise reduction. I would say 25% noise reduction if that makes any sense.
I think I bought the insulation from these guys:
http://www.sportcompactonly.com/Insulation/Interior/performance-parts_c-106_pl-2611.htm