Plumb bob, stringline, locate the centerline of the chassis. The center line of the drivetrain, from the front of the crank to the pinion, should be parallel to the chassis centerline. If going in on stock mounts, it will be about 1 1/4" to the right. If using motor/mid plates, lowering the engine will help clearances at the bellhousing. Putting a hole in the floor with a removable tunnel to fix clearances and access to bolts will make life easier.