We need to get 70 bells before world 6. There are 78 bells total that are reachable before then.

5 of them are in bonus levels. That involves beating a level a second time for the secret exit, and then beating the secret level ( + level completion fanfare x2 and watching the map load the new level). They are all extremely slow. The fastest is 1-?, which takes at least 40 seconds. So all 5 of the bonus bells are out.

Next there is 1-6 and 2-2. These bells require Brutus/Shinji, and we can't use the cat swap glitch until 2-2 is complete. So both of these bells would require backtracking. Neither level is particularly quick (both 30+ seconds), so they are out.

This leaves us with 1 other bell that we can skip throughout the run. Which bell is it? I timed out a few of the options:

2-H Costs 7.06 seconds

2-7 Costs 17 seconds (Clipping through the wall underwater vs picking up the bell with menu storage and fast cycling the squids)

2-8 Costs 9.9 seconds (+ cost of buying / activating jellyfish) (Assuming you sneak past the boss)

3-2 Costs 7.2 seconds

3-3 Costs 7.7 seconds (Assuming first try menu glitch)

3-H Costs 10.7 seconds

4-4 Costs 4.1 seconds

4-6 Costs 12.2 seconds

5-3 Costs 6.12 seconds (Assuming first try menu glitch) (Assuming you menu storage out of the cave)

5-4 Costs 10.99 seconds (Assuming good cycle)

5-7 Costs 6.71 seconds (+ cost of buying / activating penguin )

So 2-7 is the clear winner for bell to skip. Though you could make a reasonable argument for skipping 5-3 instead given that it is ridiculously difficult (Menu Glitch + Menu Storage while crossing the lava pit and grabbing the red coin).