I was trying to remove the rear drums on a 2006 Tundra & I stripped the 8mm threads used to remove the drum. I manualy backed off the star wheel, cleaned the hub area & soaked everything with Kroil & then tried to remove the drum using two 8MM bolts. The threads where rusted & I stripped them!!
Tried to retap with a larger size 5/15NF but the tap leaves a small section not threaded.
What do I do now??

If I am reading this correctly the bolts are ok but the threads on the holes are stripped? I am not sureif this applies to this situation or not but you should be able to oversize with 9mm drill bit and re tap with 10mm tap. Most toyota bolts with 10mm thread are 1.25 pitch. Hope that helps.

Thats right the bolts are OK it's the thread on the drum that's stripped. I tried to retap the hole slightly larger using a 5/16 tap - I do not have a 9mm metric bolt or tap- BUT even when I use the bottoming tap it leaves a section not threaded. I guess that I'l have to try and grind the end of the tap to allow it to go deeper into the hole

I went to a friends who had a bench grinder & modified the tap to allow it to cut closer to the end & was able to retap the holes. The drum is now removed BUT i now have another problem. The automatic adjusting lever is not there!!

Cant help you with that. The troubles you are having is what always happens to me. That is why I take my vehicles in for most service.
There is a guy here, Mustang67408, he is a toyota mechanic. Try and pm him. He can probably help.

Just go to the parts store and ask for a rear drum hardware kit. They usually have have the entire adjuster mechanism, including the lever, circlips, etc..