The real problem with Monolith would be what to cut for it. If you got a Chalice at 2 to stick, mana is probably not your biggest problem.

Why even play it over Petal? It accelerates you by exactly the same amount. Chalice at 0 is even less of a problem than Chalice for 2 as both cards are really only good t1-2 to ramp you to 3-4 mana that possible crucial turn earlier.
Monolith stays behind for Smokestack and allows sandbagging mana for a big play, and we don't need W t1 anyway. I'd say Monolith is better if you have BSA and worse if you have post-board Suppression Fields (or anything else that costs 1W); but the similarities to Petal make me vary - 'cause Petal sucks.

The real crux, what to cut? You can cut big beaters; you can cut lock pieces (replacing A with B when is only useful if you have A), or you can cut lands (weakening diamond). I'd start with big beaters, frankly - for experimentation purposes only.