i often tend to think in terms of non pushrod engines.
i was assuming the rockers were still on the car. if air is coming out of the carb, it is coming thru or around the intake valve.

i am not familiar with pushrod engines as to whether the rockers can be removed all in one shot or not. someone else can tell you tho', i'm sure. you're going to have to remove the rockers anyway to get to the springs. you may be able to just remove all the rocker arms, and the valves should all be closed, in which case there is no need to determine TDC.

sorry, sometimes my brain works in circles. (and sometimes it doesn't work)

you can determine TDC compression with the rockers on, by putting your finger in the plug hole and turning the engine until the pressure pushes your finger out. you should also be able to watch the rockers pivot up and down to see the valves open and close. when both valves are closed, you should have very little leakage. it is best to keep a renewable air source hooked up in case you lose some pressure while changing springs.

an alternate method to air pressure is nylon rope fed into the cylindar when it is near TDC, and then turning the engine to TDC. the rope will hold the valves shut. be sure there is a few inches of rope sticking out of the plug hole. (this method can also be used to help loosen a crank pulley when all else fails).