If it were me I'd calc the second gear ground speed at say 1500 engine rpm(just barely over idle) and see if that high a MPH was acceptable before I did the 1st gear/reverse gear conversion. I think you'll be surprised how high second is compared to first. It wouldn't be anything I'd want. Now, IF the second thru 6th were custom ratios that had second lower than stock and 6th slightly higher than stock and a more favorable split between them they'd really have something.