narrowing rear question

Sell the factory A body rear. Grab a cheaper B,C,E or truck/van rear and shorten it. If you have axles and SBP brakes for the A rear it's worth a bit of cash to someone that wants a factory piece.
Get the wheels and tire you want measure BEFORE you shorten the housing. BTDT and had to put spacers or run a different wheel because backspace wasn't correct.
Yeah starting from a Monaco type rear will pay dividends. When the bearing receivers go on they take up quite a bit of space, so you gotta be pretty accurate in the measuring. Better it is, to move the springs first and obtain the final tires and rims, as previously stated,by member; crackedback.