I have a 64 lemans convertible project car. There are some rust holes in the frame that I could fix but wonder about starting with a better frame. Are the conv. and hdtop frames interchangeable?

No. While it may "fit", the hardtop/coupe frame is not boxed. Convertibles had boxed frames for added support (missing the roof). Any GM A-body from '64-'67 will fit. Fix the ragtop frame or find another ragtop frame.