Thinner mounts wont gain you too much, without setting the cab right on the frame. I did a 1 1/2 " channel job on my 69.....it dont sound like much, but she also has a 4" chop top. You run out of room in a hurry, if you arent careful! My method didnt booger the complete cab, but just the center section of the floor. Lots of work. but little cost out of pocket. I agree with XXL....talk with Lotik on that 3" stock floor body drop that he did on his truck......slicker n chit, & he didnt loose any room inside the cab (as far as seating). There was a good amount of frame work on that drop tho
best of luck,
L