I don't know if the Marinco 220v 50amp is the same as any other 220v 50amp twist lock. I would get go ahead and get the marinco with boot and locking ring to keep things water tight.
Most likely if it was me and the location was permanent I would cut the cord to length keeping the female twist lock and putting a new male end on it. Then later if I ever needed the extra length I'd put a matching female end on the left over cord section.
If the cord were ever stolen I would still be able to put the right female twist on on the left over cord section.
