Well, it's actually what some guys do. Some will use R-22 because it's cheaper and easier for leak detectors to pick up. I wouldn't change the drier until after doing the air test though, most compressed air is very damp and will ruin a new drier. Replace it once you've got the leaks sewn up, it's good preventitive maintenance for the a/c anyway.
I don't know how you plan on inserting the air, but if you've got a way, more power to ya. Try to get 175# in it (not running - don't start with air in it), or as much as your compressor will push. Don't worry about diffusion, even a latex kiddy balloon will hold air overnight and your a/c hoses are much thicker than that, I hope!
BTW, when you release the air don't just let it rip, ease it out or the high flow will siphon your oil out and make a helluva mess.
Evacuating and checking to see if it holds a vacuum is good to do, but not as effective as having the system under pressure and everything being 'pushed apart' vs. being 'sucked together'. Checking if it holds a vacuum can help a tech find a big leak before he wastes time and refrigerant. It helps, but it isn't definitive.
If you opened the system while r&r'ing the engine you should replace whatever 'o'-rings were disturbed - that's probably going to be your only leak(s), unless you 'persuaded' a metal line or poked a screwdriver through the condensor. Tip: take whatever 'o'-rings you need to a shop that does a/c and throw them a couple bucks for new ones - the parts stores will usually try to sell you an entire kit. Try to get the green 'o'-rings, while not absolutely necessary they swing both ways and have a slightly larger thickness for a better seal.
One last item for my final chapter
, when you have it evac'd and recharged, have them toss an oz. of dye in it, if you ever spring a leak in the future it makes it easier to find. Even Detroit is finally catching on and most new cars have it installed at the factory. I never send one out without adding some.