i would use your 57 and 421 together , side by side and touchining each other.
tape them together if you like. so they look like this ll .
put them right up against the grill not straight on , but at an angle maybe between the center crease and the side . i usually have my second out there with headphones on slowly sweeping the mics and yell out when it sounds great.
first though , i'll have them roughly sweep each of the speaker cones to find which speaker in the cabinet sounds best. even 1/4 of an inch can make a huge difference from aweful to awesome. may i also suggest that you use a spliter after the gtr out so you can print a virgin di track - just in case you later decide that the tone you went for wasn't right for the song , you still have the performance you can always re-amp it or use amp farm or whatever.
hope this helps.
s
