Rallye clusters have the tach, speedo, clock, and the 4 piece gauge (oil, temp, amps, fuel)
The empty spot on the standard gauge cluster (which is what is sounds like you have) was actually for a clock.
For a manual I might choose a tach over oil pressure, but it'd be a close call.
Depending on your budget, I do believe there are companies out there which will make you a multi gauge that fits into the spot. It would be a one off custom job so bring your wallet