You spread that frame WAY more than necessary. Are you sure that the frame started at 126mm? That is the correct size for six and seven speed freewheel hubs and cassette hubs made before the early 90's. 135mm is the modern MTB standard. A 5-speed freewheel should indicate 120mm rear spacing.
As for 8-speed freewheels, they do exist, but they're a bad idea, particularly on a touring bike. They have a well-deserved reputation for breaking axles under all but the lightest riders. Avoid like the plague, unless you install them on a hub with an extra thick axle (like Phil Wood).