How much work are you doing yourself in this conversion and how are your fabricating and programming skills? What budget are you targeting for the conversion?
Gen 2 is a straight bolt in. You can add an auxiliary OD unit to make it long trip friendly and there are plenty of bolt in items to make it fit and function. Gen 3 will require mods to fit. Not all off the shelf parts fit correctly. If you stick with EFI, then you have sensor, wiring and programing to address as well.
Cost wise they may be a wash. Gen 2 are pretty expensive up front, but won't require an long list of custom stuff to fit and work like Gen 3 will. 5.7 Gen 3 can be picked up pretty reasonably since they can be scrounged out of late model cars and trucks. Not sure how pricing and availability is with any of the 6+ liter versions.
Value probably will favor the Gen 2 since it was an option in a 70. However, to the right person, a Gen 3 conversion may carry value. If you ever need to unload it fast, the Gen 2 will sell quicker, IMO, than the Gen 3 for these reasons.
Both have been done. Browse the board and see some details.