You are hitting on all eight here with this idea.....
I have done three cars this way and it is the best way. As far as width on this project build. The most important measurement is between the inside to inside of the front brake drums. Write that measurement down and don't lose it. Find a GM front frame that comes within plus or minus half inch. The exact measurement you require upon completion can be obtained by negative or positive dishing the wheels.
The best place to start is to take some butcher paper and hang about 4 feet up on a wall and take the measurements of the original frame and write them on the paper. Draw a base line for the original frame on the paper, insert all measurements onto this drawing. Make sure you record the wheelbase measurements exactly and get some good crossframe measurements also. Those will come in handy later on. When you have done that get another paper and hang it on the wall and project the height, etc. for your build on the new frame...... Important MEASURE 3 TIMES CUT ONCE!!!!!
I always retain the original center section of the car and use it over, because that allows you to set the body back on and have true mounting holes to line up to. Do not try and build the rear subframe until the front subframe is done. Then you can use the front section measurements to keep the wheelbase and cross rack straight.
Once you have the rear coilover section in place, then cut the original rear frame section until you can make it fit back to the rear of the new section. Then all the bumper, gas tank and body bolts simply drop back into place.... You will have a sub with an all new ride!!!!
Probably a late 70's or early Caprice sized chassis will fit it.