Mark , I have been driving my '40 with the original 216 for 16yrs and done huge miles in it. There are many on this forum who do the same.
If your truck drives nice now , what are the chances of it having a problem tomorrow?
I think using it as it is and monitoring its performance ( ie. listening to the special sounds it makes , feel the way it is on the road ) You should expect great pleasure and economical use, no point dumping more money into another engine that is actually unlikely to be any more reliable than that which you already have.
Just remember it is old and treat it accordingly.
Happy travelling
