86mm standard bore size.
It's not the head that gets rebored, it's the block - the cylinders.
All else being the same, an overbore will increase the CR as you are sweeping a bigger volume into basically the same size combustion chamber.
There are many different opinions on what CR you should use. It seems to me that the old school way to get power out of engines was to drop the CR and shove lots of boost in. These days people want better response so they tend to go for higher CR with the result being that you have to be more careful with your tune and perhaps keep the boost a little lower. RB26's start at 8.5:1 and RB25's at 9:1. I dropped my RB25 to 8.75:1 and the response is stock like. It has been at times difficult to tune but not impossible and any decent tuner should be able to deal with it.
You will see GTR's with big turbos sometimes dropping the CR to 8.2:1 or even occasionally 8:1 but these are usually big power cars, not the sort of thing you are after. That said, the Gibson GTR way back in the day apparently ran 9.4:1 and big boost from smallish turbos and ran ok. Maybe it was the datalogging and such that kept it alive. I don't know.
If stock engined GTR's can happily bolt in GT-SS's and run 300awkw then your rebuilt one should have no issues at the same CR. I also don't think that CR should be a problem at 350awkw given appropriately sized turbos.
One last thing; I've been told as a very general rule of thumb, if you drop the CR 0.2 then you need an extra 0.1 bar of boost to make the same power. I think that's how it went.
Oh, cams, yes those cams will bolt straight in. They are Tomei Poncams Type B. Cam gears wouldn't hurt but I don't think you'll be crying without them.