newby needs help

hello i have a 1976 volare that has been in the family since it came off the line at my great grandfathers plant. i have since moved it out to vegas and have some transmission issues. im looking for some advice before going to a shop. it has started to slip and was woundering if i should just do a rebuild or up grade what should i do? it has a 318 with a three speed auto. i have done a little engine mods plus 355 in the rear. thanks james