Sure, there are plenty of options of 6 speed boxes, that will bolt up to 601 thru 606. No modifications required of the boltpattern, just match the right flywheel and off you go.
It's the mounting in the vehicle, that will need to be modded.
711.651 version found in both Sprinter and ordinary car range, has these ratios.
There are various ways that you can use the extra gear.
If you want a comfortable cruiser. Change your diffs to 4.4, 4.1 or even 3.72
Enjoy a normal 1st gear and benefit from the closer ratios in 4-5-6 as your engine will feel the changed end-gearing.
If like me, you have a modest 120 hp engine, keep the diffs at 4.9 and enjoy the added possiblities of a granny 1st. Normal driving is done in 2-6.
Mine is a adventure touring vehicle, where the low 1st is icing on the cake. You don't NEED it, but very nice when it's there