Doing the same right now, only I am not mini-tubbed. I am using 10"/3.5 backspace rims. Total assembly width (mounting surface to mounting surface) was 54" and gave me about 1/2" to the springs. If I need more I will use spacers. You will need to know what brake package you are going to run as Cass will need that for the final axle length calculation. Some of the disk brake rotors are a little thicker than a drum.

The basic calulation for housing width is final width minus 2.4" per side (axle flange past the housing end), minus the rotors/drum thickness (+/-.250) per side, minus the backing plates/disc mounting brackets (+/- .125) side. The center section is about 17.5", so substract that from your number from the first set of calculations and divide by two, that is is your tube lengths. That's if you want equal length. A dana has I believe an 1 3/8 pinion offset.

As far as cutting the lip, stay inside the spot welds and mark your line with a piece of painters tape that you have stuck to something already to get as much tack off of it as you can. Use a body saw. You can get them at harbor freight for $10-12. Get a fine tooth (32) hack saw blade and cut off a portion the same length as the included blades, make sure you have the points of the blade facing the saw, so that it is cutting as it moves back to the saw. It took me about 6 minutes a side doing it like this. After that I hit it with a sanding wheel on a drill just to knock off the sharp edges.

Remember also that the Dana is an 1" longer than an 8 3/4.

If your thinking about disc brakes, I should be getting some prototype brackets and rotors from Cass next week. The brackets are bolt on and will allow the use of the GM metric brakes. So far it looks like the whole set-up (calipers/rotors/brackets/stainless steel flex lines) will be about $220 to my door.