Hey Tony. I have an original mono leaf car with the rearend still in it. I am going to a 12 bolt within the next two weeks and it is set up for multileaf springs. I have the multileafs on the car already and it rides fine. Whether or not it will/would hold up for the life of the car, I do not know. NPD advertises a kit for cars such as yours and my own. They give you the bolts and the pads that you will need to the job nicely. If you do not plan on changing the rearend eventually, I would at least think about changing the perches over to the correct multileaf perches.
1969, X-44, Export Model, Hugger Orange, Std. Int., BC Rear(soon to be a BT 12 bolt), PDB, GMPP 350HO, M22 Supercase, SSM Lift Bars, AirGap, Holley 750, FlowMaster American Thunder Exhaust, FlowTech Headers