I used 4-1/2 leaf standards on my 68. If you want to fit big rubber under there then I wouldnt go with the mid eyes.I use crappy 205 wide street tires right now but the leafs dont wrap or give wheel hop. I bet with drag radials or slicks it would be a different story but I dont intend on ever running with those.
The car no longer sags and feels firm but still comfortable. I think the 4-1/2 leafs are good because you get the ride of a slightly stiffer car and the firmer launches of a 5 leaf spring without any harshness. I bet 5 leafs would also be good, but any more might feel harsh. Also upgraded to gas shocks (just got the cheapy ones - grab a track). No rear sway bar... yet...
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'68 coupe 289 C code
66 heads, edel 600cfm carb, performer intake, dual exhaust