You have already had a bunch of great advice. I will just add to it a little.
35lbs of boost on the stock turbo is way out of its efficiency range. Most guys see 24-30lbs. The only reason I mention that is that you might not see a huge boost increase with a new turbo. 35-40lbs of boost is plenty for 600hp.
190/30s will be a little limiting on the nozzle size... but still a very good sized injector IMO. Lots of guys running them and really like them. They seem to make very good clean power. It is possible to hit 600hp... but I would not bet on it. I have ran 175/stock, 175/75, 285/75, and 190/30... my favorite combo so far is 190/30... But I do a lot of Daily driving in a big metropolitan area and plan on doing a lot of towing.
Like was mentioned before... getting rid of those 37s will really help.
It sounds like you are leaning towards non-vgt turbos but you asked about drop-in turbos. Let me know if you have any questions or need any pricing on some drop in vgt turbos.
We sell a lot of 190/30 + KC 64mm stage 2 turbo combos (drop in turbo). Makes lots of power, really fun to drive, runs very clean, and can still tow anything... BUT... you are probably going to be in the 530-580hp range.
190/30 + KC 68mm stage 3 turbo will get you closer to 600hp (550-600hp) but you will still be limited on the fuel side. Also it gets really hard to tow big loads up any grade once you get into the stage 3 68mm vgt range.