I am replacing the old thumbhole stock and handguards on my Mak90 with a nice 4 piece walnut ak kit from iroonwood design, but i cant seem to get the new upper handguard on the gas tube. I cut the old one off so it wasnt tough to remove. HELP!

You have to twist off the hand guard.  look at the end of the gas tube that is locked into the sight base.  It kind of looks square right?  Hold the gas tube in your left hand and put a wrench on the flats on the other end and give it a twist.  You need to rotate it 180 degrees, so the hand guard wood would have been pointing down.

You slide off the old wood and slide on the new wood.  Then rotate the square end of the tube back 180 degrees.  No need to cut off any wood.

You may need to lightly trim the ridge where the wood fits into the gas tube ratainers, but that is all.

Here  and here are links that show how to do it.

Go to google and type in: ak hand guard removal

if you need more info.