Hello Mr. Michalek,
If you do plan on rebuilding the engine, then it wouldnt be a bad idea to use synthetic oil. They say if you use synthetic oil when the engine is new, it will help the engine live longer sort of speak.
I would change it at least twice a year even if you dont drive it too much. I understand what you mean about sentimental value, sorry to hear about your father.
Regular oil dirties up, contains wax, and just doesnt really protect better than synthetic when it comes to cold start, etc. You can visit http://www.mobil1.com
for more info. If your going to restore your car and want it to be the best, you mine as well invest in that little extra and have the best oil in there. The only concern is I dont know if you can use synthetic in an 73, I would imagine you could though.