It looks like you may have 35-0-35 centre tap winding or do you have two separate 35v secondaries?
Either way, I would look at taking turns off, if C.T. winding you just need to reduce one of secondaries, this will still give you 20amps at 24vac.
The current capacity will be governed by the wire dia as well as the VA.
to reduce around 11vac you will most likely need around 22 turns off.