Look at it another way, doing the MCC using a 734 won't harm your chances. It will also be useful (if you do some preparation) for a Sim Assessment, which may very likely be in a recent(ish) 737 series sim.
You can also draw (as much as humanly possible!) on that experience when going for that 737 job about how "educational" the whole experience was (don't b*ll!!!!! too much though).
flash