"I want to try and keep it as original as possible."
With the price of fuel as it is and since it only seems to be going higher I would keep as small an engine in the vehicle as makes sense for your intended use. If you will be pulling a loaded trailer then by all means go for a bigger engine.
For daily transportation to work, small engine, etc.
I would suggest that you check out some books from your local library on engine swaps for Chevrolet. If they don't have any in stock they should be able to Inter-Library Loan some in for you.
No, a replacement engine will not have to be from a 1969. Chevy small blocks were nearly the same for many a year.