If it's the same sheet metal as our 1983 and 81, 5-1/2 will drop in. 6-1/2 with a little metal trimming. 6X9's with a fair bit of carving.

Minimum fuss and hacking - use the 5-1/2's and place a 8" or 10" sub box in the trunk with a foam rubber ring sealing the speaker cone to the ski pass-thru hole. Drop the rear armrest and BOOM. Unless you want to set off the neighbor's car alarms, a fairly wimpy 4-ch rear amp with two ch to the 5-1/2's and the other two bridged to drive the sub will give all you need. You don't need anything bigger than 5-1/2's on the shelf with the sub.

My son's 81 has something like this setup - you can hear him coming a long way off.
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Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)