What rear brake shoes are you using

FWIW: Rock Auto does the same thing. The part # for the '67 10" drum is the same as for the '68 10" drum.

It is possible that the backing plate was changed between '67 & '68 in such a way that the shoe's position regarding depth of insertion into the drum was changed. That could account for the difference in the width of the drum without a corresponding change in the width of the shoe.

I don't know how much room there is to work with inside the drum. Suggest putting a blob of modelling clay on the outside edge of the shoe and put the drum on. The drum will compress the clay on the shoe. Pull the drum off to find out how much free area is available by measuring the depth of the squashed clay against the shoe.

Good luck.