I have lowered my 61 a bit, not slammed but pretty low.
The rear was done only by adjusting the suspension arm angle, and the front with the bolts, and slightly enlarge holes for the adjustment.
The car rides ok, besides some very tired front shocks, that I am now replacing.
I recently crawled under the car to so the geometry at rest, and noted:
The rear bump stops are pretty close to the chassis, about 1.5" as I recall it. It would be easy to modify the rubber pads, but looking at the axel tube to chassis clearance that does not see to be a good idea I recall about 2" of clerance here.
For the front I dont have the rubber stops on there, as they are missing. (need to order a new set), but the the steel "ears"of the bum stop mount seem pretty close to the trailing arms, 1.5 inc. or so. Should the ears be modified along with a rubber blocks?
What have you guys with lowered 356s done in this regards, and how much clerance is neeed for suspension travel?
For reference, this is the amount of lowering we are dealing with: