This is basically what you'll see with the heads removed (just not this clean and lacking the notches in the pistons):
Note the two plastic trays with a single screw in the center fastening them to the block. These contain the lifters and keep them oriented to the cam lobes (keeps them from spinning).
Remove the trays, and most likely, the lifters will come out with them. In this pic, the trays (and lifters) are out and sitting on top of the engine.
The deck thickness of the heads can be irrelevant to pushrod length, but there are a lot of other variables involved too, so if changing heads and/or cam, it's always best to measure for correct pushrod length for your desired lifter preload. (this dimension can vary depending on lifters used too).
Comp Cams makes an adjustable pushrod for the purpose of measuring. It's like a $20 item. I have one if you would ever want to borrow it, just shoot me a PM.
