Call Deaver Suspension.
They build new systems, and I have talked to them before about my '68 K20.
www.deaverspring.com
The deal with the 67-68 is that the OE springs are wider at the shackle than the 69-72 versions, so in order to use the 69-72 spring you will have to use a steal collared bushing and a spacer in each side to center them in the shackle.
I am shopping for new springs right now, but I am looking for 2" or 2.5" lift.
Deaver systems only offer 4" and up, so I am looking else where like Skyjacker at the moment.
Member "Mt Man Chad" did a little write up on using 69-72 springs on his burb.
http://67-72chevytrucks.com/vboard/s...=456901&page=4