At a min, I would pop all the freeze plugs and rinse everything out and clean the block through the freeze plugs (with water hose, screwdriver, etc). This is best done with ththe engine on a engine stand.
I did this on my engine (it was out when I bought the car) and was able to clean out a bunch of rust from 20 years of sitting (as well as what looked like original casting sand). It really only takes a couple hours to pull the engine if you have the tools/ space. If the engine ran well, a quick cleanup and gasket change could be done over a long weekend. As mentioned, you always run the risk of project creep.
66 Coupe Project 289