I too have the 4.5 leaf in my 66. The ride is nice but not harsh. I believe it was an appropriate choice over the five or four leaf. You may also need consider the eye location; stock, mid-eye, or reverse-eye. I initial bought stock, and my backend sat so high you could has sworn I had air shocks. ended up settling on mid-eyes.

 

I am with JONK67. I love the 4.5 leaf. I bought the mid eye but i wish i would have went with the reverse eye. It could easily sit 1-2 inches lower than it is now. I am running the KYB gr2 shocks and i dig those also.

 

I say go with the GT springs 4 leaf.