Considering the 65 Skylark GS used a three speed top loader and the bellhousing is drilled for that Ford product, did Ford ever make a five speed with that pattern? I doubt it as I have never seen anyone comment on putting a Ford five speed in a GS, but just hoping.
the short answer is none but the T-5 transmission and it's later variants may work if you try really hard. I'd wait a bit and see what becomes of the McLead 5-speed that is set up as a bolt in for a Muncie.
Tremec makes the TKO transmissions, top loaders with overdrive and several different gear shift locations. McLeod is supposed to be coming out with a 5 speed that is about a grand cheaper than the TKO but it got held up about a year ago. http://www.americanpowertrain.com/
If you want to make it a painless experience I would go with the late Ford toploader as its a drop in replacement for the 3 spd. If memory serves its not too common but they are cheap. Still cast iron case and you can get a new hurst shifter to bolt right up and reuse you clutch parts. I cannot offhand remember what it came out of. Check with the Ford guys. I do remember selling mine for like 150.00 when I was selling regular top loaders for 800.00
