The triangle is a point on the flywheel used to check for timing gear wear and doesn't relate to ignition timing. The ball is your factory timing mark, probably at 4 degrees BTDC as your shop manual states and is a good place to start.

You're gonna have to play it by ear a little on this one. The timing can be advanced until pinging is heard, then backed down just below that point. On my HEI the timing ball is not visible in the window unless I look downward into it. If you knew the diameter of your flywheel I would think a guy could mark it off in 5 degree increments pretty easily but who knows where your engine's "sweet spot" is. Happy hunting!!