The front mount only goes on one way, but there is some variance in it. You are not going to be able to set your pinion angle until the engine is in the car. Like I said, I’d tack it into place at most if it were me, but if you want to go for it and finish weld it, have at it, you just have fewer options when you actually have to set the pinion angle if it needs adjustment.
Without the engine in, you don't know what your engine angle is going to be, and therefore what your pinion angle will need to be. This is pretty critical, because if it is too far off you can get vibration, driveline wear etc.
If you finish weld the front mount now, you'll still have some adjustment, but have fewer options to fix any issues.
Yes you can use an adjustable sublink, but I would set it to the stock length for mockup, again because you don't know what adjustments you are going to have to make down the line. Plus this will also affect the rear camber, and adjusting that later could cause other new problems as it twists the subframe.
