Decisions, decisions!
Your easiest swap will be 600 to 800, bolts right in same as 600.
The only differences from a stock 800 will be one less heat exchanger, handlebar riser, color, and 12gal tank. Carbs are the same as 600 just larger jets. Don't change the cog gearing even AD Boivin went back to original 503/600 gearing after problems with 2007 Top Gun's fourth pully and longer belt. The heat exchanger could be retrofitted but if you ride in deep snow and never had a problem with your Outlaw before should not with the swap.
If you print and study the manual PDF's on ADB's site you can decifer the differences notice the 503F engine mounting plate is placed higher, the block is bolted ridgidly through dowel spacers and its mounting holes are spaced different. The 6 and 800 engines are rubber isolated. The 503F engines are piston port, not the cylinder reed type, putts the carbs much higher requiring partial bulkhead (triangular steering stem mount) removal and shorter steering stem. Your 600 has extra brace bracketry which also mounts the MPEM.
Our prototype 2006/7 Top Gun 800 was simply a conversion from a 2004 600ST so it was missing a few upgrades yet never had a failure.
The primary TRA III and secondary clutch on 800's are the REV 2004 on, which requires a different diameter stronger steel secondary jack shaft. Better to upgrade to Artic Cat secondary the guys on here Sledder 2, etc. for their replacement steel upper and lower jack shafts. Put four new premium bearings in while you are at it, all same size.
Replacing both shafts with solid steel units and bearings often is a must in the long term since it seems that Eighthunnert seems to wear and break things quicker, go figure.
You could contact Whitesurf in BC for their suspension rail heat exchanger conversion.
http://whitesurf.ca/
Your suspension should be fine just set up for total weight including rider and riding style. Either the Jaws or quieter Matek exhaust is the same 600/800. If you do not need the fuel range the bulky 12gal can be cumbersome but an 800 is a thirsty beast.
Attac Altitude compensator should be installed if your new engine is not a Summit DPEM powerplant. Scram600, Sledder2, and a few others have some great ideas and retrofits for all machine models.
I would like to see an Etec 600 swap, 22mpg would be nice just think of all the weight you could lose by only needing the stock 503F 5gal tank.
600 has plenty of power if clutched right.
Have fun with your project.