The 6060 is a strong box but its ratios are not acceptable for a Cosworth.
6th is a stupid 0.5 even if you change the Diff ratio the spacing of the gears are wrong.
Below are the ratios of the latest version v a T5

2013 Ford Mustang Shelby GT5002.66:1.82:1.30:1.00:0.77:0.50
Ford rs Cosworth 2.95: 1.94: 1.34: 1.00: 0.8

You simply cant use that 6th gear its like falling off a cliff. Get that right by changing the Diff & the other gears are too short.
The T56 was the previous 6 speed from Tremec I toyed with that but realised it was not suitable.