I also would skip the dextrose. With three pounds of Amber DME, I don't know that you really need more specialty grains, either.
The late extract idea is a good one, but I would rather see you boil a larger volume. For a beer that big, you need to boil at least 3 gallons to make it work right, and you will see tremendous improvement with a full wort boil. With the wort concentration you will have on a 1.5 gallon boil, you will have serious issues with kettle carmelization and even, possible scorching. That's before you even get into hop utilization.
If you can't go out and get yourself a five-gallon kettle or larger, try to keep your beers in the 1.050-55 range, tops. Even that is pushing it.