Analog Hour Meters

If you have switched alternator sense, a second wire to your alternator (not tach signal), you could tap there. I'd say 9 out of 10 installs are self exciters and only one (battery) lead would be found.
Depending on the mfg and harness, You may find a switched B+ on a terminal strip near or close to the engine, in line of the engine harness, not connected to anything.
Since Detroits (Non DDEC) do not need juice to run, that is my first two guesses.
Interesting you do not have hour meters already installed.
Later hour meters I install are under the dash, taped to the key switch B+ that powers the volt meter or dash gauges (Usually purple wire). No need to bore a dash hole and you don't have to crawl into the ER to check hours.

Uh?

Uh??
If the engine room mechanical gauges are mechanical, there is no B+ power to them. They work on an umbilical tube from the engines. The back lighting for them may be key switched on.
Now if the engine room gauges are electrical and work from the key switch, life is good. Tap on the B+ side of the gauge.
Send us a picture of the gauges back side if your not sure.
,rc

Hang on, hang on;
Been a long day and a late cocktail to remember this.
Is your alternator belt driven or bolted on the back of the engine (direct cam drive)?
Leece-Neville (Prestolite) alternators required a key switch exciter line. Usually a purple or yellow with a purple strip (Not red stripe). This would be a good switched wire to tap on in the engine room for a hour meter.
Send a pic of the back of the alternator.