I have 2 road bikes:
One has 9 speed Dura-Ace 7700 components (50-34 x 12-27 gearing).
The other has 10 speed Dura-Ace 7800 components (50-34 x 12-27 gearing).

I will be riding in the mountains in May and want use the lowest gearing possible, a 11-42 rear cassette would be ideal. I understand I would have to use a mountain bike derailleur to do that.

Shimano claims the largest cog recommended for their rear der's is a 36T yet I know there are some that work with a 42T. Will the der below work even though it shows a 42T capacity?

Any advice would be appreciated..Thanks in advance.

Forté 9-speed MTB Rear Derailleur