AND,...That short shaft driveshaft housing assembly (DSH) can be used under a 650ss, 950ss, 1000ss, 1100ss, 1250, 1250ss, first edition 1150 & the 1350,...If your 800 4 cyl is too puny for the boat of choice. The exhaust extension plate (EEP) from the 800 will only work under the early 1150 & 1350 (maybe the 1250???), but armed with their respective original EEP from each listed model, that first edition 800 DSH, swivel housing, and transom clamps, etc can bring you closer to a variety of 15" shorty options.
The related originally long shaft exhaust tubes on these specific Mercs can be easily shortened to a short shaft application.
Some of the power head mounting studs may need to be swapped out to match the lengths required by the 800 DSH, so hang onto those original studs, if doing a power head swap.
You might find that you need to leave out/remove the (2) 5/16" studs at the aft of the power head on the (10) stud units, but the power head, gasket & DSH won't care.
The shorty 800 lower unit is originally a 2.31:1 ratio, where the bigger inline-6 used a 2:1, and the 1350 used a 1.78:1 ratio, so if you only have the long shaft lower unit for the power head of your choice, I would disassembly and ship out that driveshaft to be shortened to use in the 15" application. That way you can re-assembly your lower unit without needing to re-shim or re-set lash, etc.
Trivia for the day