Welcome to the forum
I would recommend the L31 Vortec crate engine. There are several members here that have bought that engine and are quite happy. There is a current thread in the Engine and Drivetrain section about one and where is the best price to purchase. Having the dipstick on the passenger side is not an issue and is not that close to the heater.Most name brand block hugger headers should fit as well as Hedman 69260 long tubes which are made specifically for the '55-'72 trucks. You must have one of the odd '67-'72 power steering pump brackets that also bolts to one exhaust bolt. Most all of the '63-'72 power steering pump brackets do not reach up to that exhaust bolt. Of course with headers all you will need is a longer bolt and a short piece of tube to make up the difference.