Just put a momo on my car, what I did was remove the cruise control all together as it dosen't work which includes removing the slip contact ring which mounts to the front of the combination switch. Then using a combination switch I got from a 88 ST162 Celica, I took the spring loaded horn contact out of that switch and placed it through the hole in the Supra switch and wired it back to the loom.
The momo boss kit you need is Part No. 7715 which is actually meant for Celicas and Landcruisers. This boss kit has the same splines, the horn contact ring contacts the horn contact correctly and even the indicators cancel properly! This boss kit is a bit long, it does move the wheel a bit further (like 15-20mm or so) away from the indicator stalk but apart from that its all good
And the best thing with the momo hubs is that they are collapsible, so they are legal!!!
I originally tried with a momo boss kit Part No. 7713 which also fits some Celicas, this one is the perfect length for A70's, but the indicators wouldn't cancel which drove me up the wall. If you wanna try it however, make me an offer its just sitting in the shed doing nothing...