I second the recommendation of a blown burner for your forge. The forge body is going to have a lot to do with building up heat too. Try three layers of 1" 8# ceramic fiber insulation with a thin coating of a castable refractory. My heat treating/welding forge has a large mailbox for a frame and was lines as above. I has one burner and can get up to welding heat. You could use something like refractory and/or insulation brick to block off the back and restrict the front or you could go whole hog and build steel doors for your unit. There are people who have held that they were able to get welding heat with venturi burners but I think that for that size of a chamber I'd use two burners. The only thing that is "tricky" about a blown burner is the blower. They might run a bit of money but many have scrounged used blowers for little or nothing. All you need is somthing that will put out about 60cfpm. The rest of what you need for a blown burner can ge had down at your local tool-in-a-box store or local hardware store, though here locally I had to buy my needle valve from Ellis Custom Knifeworks when I got the hose, connecter, regulator and pressure guage, which you have to buy for a venturi burner too.
Doug Lester