The lower is easy. There are 3 ways to do it. From easiest to hardest...
1. There are 2 small metal pins that keep the sheetmetal guard in place. Just notch the end of the handguard that fits into the retainer to clear the pins.
2. Remove the metal pins from the retainer. The pins have a post that is inserted into the retainer and then peened over to keep them in place. You can grind the end of the post off with a Dremel tool and easily remove the pins.
3. Remove the Front Sight Block and Gas Block to replace the retainer with a standard version. This is a lot of work and will cost you for the retainer, but it is an option. Personally, I'd go with option #1 or #2.
A standard gas tube can be shortened like HKJAY said. Just cut the tube to the same length as the original 65 gas tube. You may want to leave it long and file it for a perfect fit. To get the flare back on the tube, you can use a 3/8" socket extension. The extension usually has a nice radius transition from the shaft to the end that fits the wrench. This radius quickly returns the gas tube to a round shape when you force the extension into the tube. You can use a hammer or a vise (or hydraulic press) to push the extension into the gas tube. Then use a socket as a round form to help get a perfect end on the tube. File it to length and you're set.