Here is a photo of some common SL6 oil pans, the B-body pan is at the lower left corner, your 73 A-body should have the "087" pan at the lower right corner. Look at how much shorter the rear of the sump is to the back edge of the pan.

The 60-66 A-body oil pan has a sump that is a full inch further back then a 67 and later pan.

There is no number stamped into the early A-Body oil pans. You just have to look for the long shallow front section, the sump does not start until appx the 3rd bolt hole back.

Sorry to say that the "987" pan is one of the pans for the 67 and later A-Body cars, this pan will not fit 60-66 A-Body chassis.

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Here is a look at the pan you have next to the one you need. The early A-Body pan has a much longer front section.
FYI, the only other pan that will fit is the "369", mid 70s A & F body unit, the blue one in this pic. The front left corner of that pan comes close to the 60-66 K-frame so I dent it in a little to make clearance. (see the black lines)

I call those later F-body oil pans the "small sump universal pan" just because they will fit into a lot of different chassis. It is a good pan to keep on the "back-up engine" because in my case, I never know which car that engine may end-up in.

Watch the front driver's side corner on this pan when using it on an early A-body, it will hit the K-frame on hard acceleration. I just dent that corner in a bit and it clears. Also note that the rear section is a little deeper so it is tight with the shallow 60-65 A-Body SL6 centerlink, the V8 centerlink clears it.

The 1960 - 66 pan is different from the 67 on up pans, the area between the sump and the front of the pan is longer.
Oil pick-up tubes are also different so swap on the oil pan and pick-up tube

I call the later small sump "369" pan the "universal" oil pan for the SL6 because it does fit many chassis. (the blue pan in the photo below)

The 369 SL6 oil pan will "drop into" a 60-66 A-body but the shallow SL6 centerlink will rub on it's rear, no-sump section and the sump's edge is really close to the K-frame on the front left (driver's side) corner.
As routeen, I take a bodyman's hammer and tap some small "depressions" (dents) into these pans, even before I pull them off the donor engine... I've done enough of them to know right where to dent the pan so they fit right into an early A-Body.

Using the deeper drop V-8 center-link reduces the size of the dent needed to clear but I find that the 369 pan still needs a small dent in that area, to clear the deep c-link at full lock, right turn.

Dent the 369 oil pan Here:



And Here:

Donno what used to be on that page, and I don't have an archived copy, sorry

Aren't the oil pickups kind of unique to each oil pan?

Update: I found that content archived on an old hard drive. I've put it back up and now the link works again. Click and see.