Welcome Aboard! and congrats on your first kit!
Caliber conversions are pretty much straight-forward. You will get a few different answers, since a "basic" conversion will work, but some folks believe in going all the way. I'm a minimalist myself, so I'll chime in on the "only what you really need for it to work" methods

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First off, when you build your weapon, make sure you use a right front cartridge stop (30.06) and the supplied Israeli rear cartridge stop (this one usually still has rivet remains in most kits). I'm assuming it will be a standard, Israeli 308 build..........Now for the conversions:
30.06...additional parts-
*30.06 Booster - Opening diameter of .718", or the M7 flash hider/booster
*30.06 Barrel
***
optionalShort Cartridge Stop - These weapons are very sensitive to overall length of cartridges being used. If using milspec M2 ball, you should be fine, but if you use commercial or have variances in OAL....you may want one of these.
...that should be it for the parts. Now here's what you do:
1) Remove front and rear, left side cartridge stops/guides. Make sure you put the BH pawl, spring, and pin back in
2) Change out barrel with 30.06
3) Change out booster
4) Headspace and go
8mm...additional parts-
*30.06 Booster - same one you use for 30.06 (.718' diameter)
*8mm Barrel
*8mm Front Cartridge Stop/Guide
Here's what to do:
1) Remove both 308, Cartridge Stops/Guides
2) Install 8mm Front Cartridge Stops/Guides
3) Change out Barrel with 8mm
4) Change out Booster
5) Headspace and go
There will be some discussion on other parts to change out, but that's all you need to run all three calibers. If you really wanted to, you could change out the topcover to a 30.06, but the only rear difference is the belt feed pawl. All of mine run just fine with either topcover for the three calibers discussed. The only real difference, if I am correct, is the arm bend in the pawl, and the serrated portion that grabs the belt/links.
On the 8mm Barrel, who's your supplier? (they've been scarce lately) There have been some issues on the chamber cut that affect the Yugo surplus 8mm. Make sure you find out about that. You will want a chamber that is reamed out to handle the Yugo stuff.
Hope this helps. If you need any more info, don't hesitate to ask. Things are a little slow over here right now, but someone will chime in sooner or later.
Matt