The main difference is the pump. The early 700R4 had a 5 vane pump, and would lose pressure in overdrive. If pulling a load, the lower line pressure would let the clutch plates slip. The later version has a 7 vane pump, which eliminated most of the problem, then when it became the 4L60E, an electronic pressure regulator was added that eliminated completely by adding the "Tow/Haul" mode. The 7 vane pump is an easy upgrade on the early 700, and clutch packs from the 350 can also be used to eliminate slippage. Both are cheap upgrades.
