Toploader questions

[FE427TP]

I have a HEH-CL Toploader I just recieved,
1.] How hard would it be to convert this to big in big out?
2.] Is this even possible?
3.] Out of curiousity is there major differences in the cases between the small and big input and output shaft cases?
4.] Is big in/out needed for my situation?
it will be a 1968 shortbox Pickup and behind a 410 estimated weight about 4000lbs 3800gvw+driver. The 410 will put out just under 500 ft/lbs of torque and I may play with medium sized shots of nitrous 200horse max.
5.] Will changing the intput or output shafts require changing the gears too?

Thanks for your time,

Caleb