Welcome! Where are you located?
A 4-speed convertible with AC is definitely a rare, sought after GS.
You can use the frame from under a regular '64-'66 convertible, but be aware the GS frame has two extra gussets welded into it at the crossmember over the rear axle. Using the original frame as a pattern, a good fabricator/welder can make some duplicate gussets and weld them into a regular Skylark convertible frame though.