Was the load in the car the same each time? A early run NA 2 seater T-top was listed with a Curb weight of 3220lbs. Add ~54lbs for a half tank of gas to the 3100 and the difference between the two is 66lbs. That sounds a lot closer considering a LS engine weighs around 410-425lbs with accessories and a VD30DETT is just under 500lbs with accessories and some extra weight for turbos that the NA wouldn't have had. So if memory serves the stock turbos are ~12lbs, putting a VG30DE around 475lbs, minus a 410lbs LS1 leaves right around the 66lbs mentioned above. In order to lose 300lbs with just an engine swap the non-turbo VG would have to be over 700lbs.