hi david,
as Simplex and Super Simplex have large grates in relation to the overall boiler proportions you can safely reduce the width of the firebox.
one tip i would make is to NOT bore out the cylinders to the drawing size but to make the bores less on Super Simplex. the loco is 'over cylindered', a fault also with Simplex. bore the Super Simplex cylinders out to 1.5" dia, no more. fit top feed to the boiler, and you may also want to add proper crown girder stays or crown rod stays instead of the martin evans arrangement of strengthening the inner firebox crown.
cheers,
julian