This info is from the Green 140 manual for 1972, page 2:25, picture
in the lower left corner of the page. I'm assuming you DON'T have a B20A,
which requires a different setting; B20 B, E and F are all the same.
If you look at the distributor end of the gear/shaft you will notice the
slot for distributor drive is off center. One side of the slot is ON
the centerline of the gear. That is the side you guide from. You want
#1 at TDC, firing position (both valves firmly closed). #4 will be in
the overlap position (both valves slightly open). The line through the
slot on the gear should be about 5° clockwise from straight across. You will
have to get something long to turn the oil pump so that the shaft flat
will go into it at the proper orientation. This orientation is when the
gear and shaft extension are fully seated - it twists about one gear tooth
on the way in. Don't feel bad if it takes several tries to get it right.
When it is right, the line through the "center" side of the slot will be
just above the left (front) mounting bolt hole and an equal amount below the
right (rear) one.

(Yep, a picture probably is worth 1000 words. Better get the green Volvo
manual the next time you see one on ebay!)
--
George Downs, Bartlesville, Oklahoma, Central US