I'm putting in a new tub and there is virtually no access to the underside/back parts of the tub once it's in place (my house is on a slab and there's a furnace right next to the front part of the tub).

How do I install the drain assembly? All the instructions say to put it on the tub, then lower the tub into position, but then I'd be blindly connecting to the drain without any way to secure it with a compression fitting.

Thanks!