I think the 5 speed will be a significantly easier swap. The T-56 is going to require you to cut and make a custom trans cross member. It is significantly larger transmission.

All that said - I'd go with a T-56... but i'm biased

1970 Charger - V10, 6spd, Alterkation, Street Lynx 4 Link, Moser Dana 60, Wilwood 14" disks, Forgeline 18" Wheels (Rear:335's), ISIS Multiplexing Wiring http://www.v10mopar.com 2012 Charger SRT8