A full frame DSLR would be nice, but I don't see where it would gain you much over your current 60D. You're shooting from a tripod with complete control over lighting. If you're shooting at ISO 100 with the 60D, you won't see a difference shooting at ISO 100 with a 5D3.
The single biggest difference you will see with a 5D3 will be a wider angle of view.
Sounds like you may be better off investing in a second lens. Perhaps a 24 mm. While not cheap, there's the EF 24mm f/1.4 L that would give you slightly better low-light performance than what you have with the 50mm.