4/6 is the limit on dropping your truck without modifying the rear frame for a C-notch. Some may say that you should do a C-notch for a 6inch drop on the rear, really your own decision to make. Chris (powell72) is correct that you will need to goto disc in the front. Are your rims 5 lug or 6 lug?
Here is a shot of my 67 short fleet with the 4/6 drop (but my rims are 15 inch)
