On a push rod engine, you CAN remove all the rocker arms at the same time. Once you have all the rocker arms removed, you can put 60 PSI air pressure in to ANY cylinder through its spark plug hole and it should keep the valves closed and NOT blow up through the carb, regardelss of which piston is at TDC.
If you still get air coming up through the carb in any cylinder, then you may have a problem with a valve, a seat or binding somewhere between the stem and the guide. Of course, the car would probably run like sh*t if this is the case.
If you do need to locate TDC, do that first - before removing the rocker arms. Line up the timing mark on the balancer with the 0* mark on the timing plate. At this point, either #1 or #6 will be at TDC. You can easilly "spin" both pushrods of the TDC cylinder.
From that point, you rotate the balancer in 90* increments (CLOCKWISE while facing the engine) to go through the rest of the firing order. 1-8-4-3-6-5-7-2. You can guesstimate the 90* increment - it need not be exact.
