I think you should PM FishlessMikey about the FB. If you don't mind someone wearing a skirt makes it. Then as for the throat opening I try to keep it as low as possible so trapezoid is what I tend to do.
Making memories.
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........
if you do a round firebox then the diameter of it should match the diameter of the cook chamber. if you do that then you will use an eyeball shaped opening. Pete Mazz has made several useful calculators to help make sure the Dims are correct. you can find them in his signature...
as far as how to mount it I recommend cutting off the endbell on the firebox end of the cook chamber and plating off the end to make the mounting easy
Just my 2 cents but this would be a great question for the person who is going to do the project for you as it sounds like the correct answer comes down to cost.
They will have to decide if it it more cost effective to cut a sheet of plate and weld square or cut up a $100 tank and put a door on it. It might be pretty close as that amount of plate should be less than $100 so then it is just down to labor.
Personally, I will cut and weld plate any day as opposed to working with an old tank (with rust/old paint/fit issues) but that is just my opinion....ask the guy welding if for you for HIS opinion as it may be different. How much prep are you going to do? Running a bead is pretty cheap (if all he has to do is weld it) but if he has to cut and fit on a tank that is gonna cost you on the labor.
I really like the way mine came out except the hinges are to close the the cook chamber and it will guillotine you if you don't have it leveled right. The air vent is in the wrong location but it works nice as a window so you don't have to open the cooker. I just open the fire drawer a little and it draws nice. The drawer is just the cut off piece of tank flipped and dropped down in the fb. If I had the money I would have gone with square just for the fact of less redneck engineering.