420s are sick but they are crazy wide so you would basically have to completely remove the pedals so its no longer an MB(and be disqualified from any races or records), move the bottom bracket and use a crazy multi-chain setup to route the chain around it(which is cool cuz you can have super comfortable feet forward geometry but complicated and expensive), or if its a stretch frame run the motor way far forward with the pedals fully behind the motor which will be uncomfortable since your feet will basically be under you and your weight distribution would be effected with the motor that far up front. All methods are possible, but not ideal IMO.
On my 305 Briggs (which actually seems slightly smaller than the 301 Predator) I moved the engine forward about 1" more than I initially wanted to make room for the cranks so I can still have functioning pedals that aren't useless little 2" nub cranks. On the positive side, it will let me pull the valve cover without removing the motor now which is nice, and also gives more room for a good size gusset between the seat post and top tube.