Here are my thoughts, up for cussin' and discussin':
These trucks, 60-66, have low hanging front crossmembers and lower control arms. You can do a 3" front drop with springs. If you then use 2" drop spindles the lower control arms will really be too low due to scrub line issues for a realistically safe daily driver. A lot of olks are using aftermarket front crossmembers which solve some of these issues, but are pricey.
As for the rear you can go with 5" drop springs, anything more will require notching the frame. And if you use lowering blocks in the rear, anything more than 2" is also bad, again due to scrub line issues.
A scrub line issue basically means that if a tire goes flat that suspension components or the frame will rub on the ground. If that happens you may well be at the mercy of the god's of speed as to if you live or die.
Here's a photo of my '65. It has a 3" / 5" spring drop, stock front spindles, no rear lowering blocks, no rear notch.