On a bbc when you change timing chain do you have the engine on compression stroke TTC to align the 2 gears. Or just align the 2 marks on the gears.

The marks on the gears will do what you want.

tdc comp stroke puts both dots @ 12 o'clock-setup with both dots pointing @ each other(cam @ 6 o'clock, crank @ 12 o'clock) then spin engine 180 degs so both dots point up, then you wont forget and have dist. off 180 degs-this throws off more people than that 307/350-454 frame mount thing
why they do things(gm) is beyond me-lol

line up the dots before you remove the old gears, mark the distributor rotor location and the location of the vacuum advance can and it will be correct first time everytime.