On some of the later engines, GM changed the zero mark on some of the balancers to use timing marks in the center of the timing cover behind the water pump. There are several versions of the balancer and tabs, so it is best to always check TDC when assembling. If you look at the keyway slot in the balancer and rotate it to about 2 o'clock, the TDC mark will be at about 12 o'clock on ones that need the tab welded in the center of the timing cover. The others will have the TDC mark closer to the keyway tab. But, I have seen this on 305's and 350's and both 305's and 350's had the dipstick tube on the passenger side.
So, the only way that I could tell for sure was to run the numbers off of the block on the drivers side rear where it bolts to the transmission.
Bill M