A lot of the old thumbshifters, whilst only showing on the top that they have 7 speeds (and therefore 6 definite "clicks" when moving them), have a hidden 7th click which allows use with any Shimano 8sp rear gear system.
Just check your thumbshifter - if it has 7 clicks (the hidden one will be with the lever all the way anticlockwise) then there are 8 gear position so you'll be fine!
The front shifter isn't indexed so that'll work with anything.