[Jac Mac]

The gasket also acts as a baffle between the lube and the original breather hole in the top cover, as long as you have now blocked that original breather hole now that you have fitted the replacement breather in the tail housing you should not have a problem. However be aware now that you have no gasket that the 'feel' as you tighten the bolts will be different since there is no gasket to compress or squash.

With regard to checking ratios its quite simple with the top cover removed.
Look at the gear on input shaft at front of case next to the front input shaft bearing, if the tooth outer dia is virtually the same as the OD of input shaft bearing its a close ratio ( 2.32/1 low )....... if its about 1/4" smaller in OD than the input shaft bearing its a wide ratio ( 2.78/1 low by the look of your earlier pic of the phillips head detent screw you can just see the gear at front being noticeably smaller than brg, so it looks like you have wide ratio)

Also back in the 62 / 68 era with the Nascar stuff and T44 transaxle for the GT40 MKII & MKIV there were many more ratio options for these box's, about six different options IIRC