The battery is a no brainer it should be well sulphated by your description. Have it properly checked out.It has sat around far too much over the years. Valve adjustment if no records of one in the last few years. Or at least a valve clearance check.

Establish there is enough fuel pressure in the injection pump supply to enable overflow into the return line. Very low injection pump supply pressure will hinder easy starts to some extent. In fact if you like the car you should get a liquid filled gauge and get a running pressure of at least fourteen pounds present in there.Around 18-19 pounds is even better.

Read the extensive thread on the relief valve. It may be very important on the 616 engines in my opinion.