There is a ball embedded on the foward face of the flywheel and you can see it through the round opening in the bellhousing just above the starter (there is a pointer in the round opening). Clamp your timing light to the number one plug (or number one plug wire) and point the strobe light at the pointer on the bell housing. Adjust the distributor until the ball lines up with pointer (at 500-750 rpm, depending on your engine, transmission, and carburetion). When setting the timing, you should also disconnect the vacuum line from the intake to the vacuum advance on the distributor (and plug/seal the line, so there is no vacuum leak at the intake side).
Oh yeah, set your points/dwell before setting the timing.
Regarding valve lash adjustments, the factory manuals state that you should bring the engine to operating temperature and then run for and additional 30 minutes before setting the valve lash.
All truck engines (235/261) had mechanical lifters. Some truck engines might have been converted to hydraulic lifters. Many automobile engines have been put into trucks and most post-1953 car engines had hydraulic lifters (but, a car engine might have been converted to mechanical lifters).
I set the valve lash on both my 1954 235 and 1960 261 to 0.006" intake and 0.016" exhaust (hot engine). GMs manuals for the same year give different lash settings (also, the larger trucks had wider lash settings: 0.008" intake up to 0.020" exhaust).
Tim
