A 85 amp solenoid is on the low-side. But might work for 35Ah batteries.
A minimum of 125A is better. And will probably work in 35-60 Ah batteries.
But 200A+ is very much preferred.
The challenge about the amp rating for a solenoid, is the in-rush amps. When you first connect a depleted house battery, to a fully charged starter battery - then the voltage difference will want to be equalized in an instant.
So at the time of contact, there will be a good size spark happening inside the solenoid. At the contact points.
And for the first few seconds the amps will be as high as possible. And may continue to be at the high end of two digits some minutes. And then things will settle at a much lower rate, for the reminder of the charge time of the house battery.
But the internal contacts of the solenoid, will need to be able to survive that big initial spark, as the contacts first connect, and 100-170+ amp starts flowing, even if for only 10 seconds.
This takes some serious copper flanges, with some serious contact points, to repeatedly survive that kind of "abuse"/"normal operating conditions".
The next thing is, that the coil needs to be able to hold these contact points together for hours on end, without the coils overheating.
So beefy, in every aspect, is the way to go, when directly connection two batteries only via a relay/solenoid.