What you bought should be perfect. Tell your mechanic that the engine is just the same as a 1970 VW Beetle. Drain plug is in the bottom of the sump. Re-fill to just under the “full” mark on the dipstick. It should take somewhere between 3 and 4 quarts (all our engines are different so I’m guessing, here). You could also buy a 1965 - 1970 ish Bentley’s VW Service Manual (usually on Amazon) which tells you (or your mechanic) all this stuff and everything else about maintaining your car.
BTW, you may, and this is a long shot with a 1641 cc mostly stock engine, have an oil filter that is external to the engine. Looks like a can attached to the car frame, about the size of a 4 cup Measuring cup, usually orange or white with big hoses going to it. If you or he finds that, it should be changed with the oil. Just match it up at a NAPA auto parts store.
if you don’t have that one, no worries. There is a straining screen inside of the sump that could be changed if the mechanic is in to that sort of stuff. You’ll need two sump gaskets to replace them once the screen is cleaned. Here’s what they are:
(That’s a link)
If you don’t clean the screen this year, make sure you do it next year and it'll be fine.
And Here's a link to the Bentley manual on Amazon