Setting Driveline Angle Subframe as Reference?

[OBeer-WAN-Kenobi]

Can I go off of my factory subframe as a reference and set the angle of my engine and transmission outside of the car?

I need to fabricate a transmission mount for my LS1 T56 combo in my '69 Camaro and I was wondering if I can just go off of the subframe for now? What I plan to do is favor it more downward in the rear so I can just make some spacers to put under the transmission mount if I need to raise it a bit when it's actually installed under the car. I'm figuring this whole operation would be really easy to do at the moment since the subframe (though I will probably be getting a replacment sub) is out of the car.

So, if this isn't a dumb idea....If I level the sub frame rails, what angle should the motor and trans be at? In other words, what angle should the motor and transmission sit at in relation to the subframe?

Thanks!