I went with espo 6 leaf 340 springs 1 inch over... car sits nice and rides good. I think it was around 330 shipped for a pair. Great people to deal with and matched set of springs.
my 68 cuda is 2x3 sub frame connected with a cage... the entire floor was cut out from back seatish area to gas pedal and replaced with aluminum. it was done in 1970. someone was planning on making a full race car out of it