Welcome fellow GS'r.. You will most definitly need to rejet for pods and open or modified exhaust... how much and what recipe, every bike is different... "2" 79 gs 850s with the same pods and exhaust may require different tweaks... The good, your bike has VM26 carbs, much easier to tune than later model CV carbs...
I would advise doing this....
1. check valve clearances and reshim as necesary.
2. pull carbs apart, get new bowl and top gasket (Z1 Enterprises) and a carb oring kit (cycle orings i think is the vendor), unless brass bits are trashed you wont gain anything from a rebuild kit... soak carbs in a good quality carb dip 24 hrs each. blow out with compressed air, all passages.
3. check carb to head rubber boots.. hard and cracking? replace (about $130 for a set of 4)... soft, no cracks, replace oring.
4. exhaust, run mufflers for now so you have some back pressure and your tuning will be sooooo much easier... its better to get it running as close to stock as possible then make small mods that you can then understand the effects of the mod...
go here, download a manual and read... read, and reread all the tutorials on the site..... wealth of knowledge...
http://members.dslextreme.com/users/bikecliff/