First off, Iron Defense is entirely unnecessary on Shuckle there: Power Split doesn't take any stat boosts into account, only the underlying stats. I still don't know if it even does take Power Trick into account (Shuckle is worthless in the scenario if it doesn't), which is why my most recent plan above (with a runtime of 32 turns) provides separate contingencies for 459, 525, and 613, each representing the maximum attack stat that the respective holder would be able to splice onto Pikachu.
Second, a Ditto that transforms into Pikachu will not get a boost from Light Ball--to the contrary, if Pikachu were able to transform into anything (which is not currently possible under G7 rules with legal sets), Pikachu would still get its Light Ball bonus no matter what form it takes on. This is a non-issue in any case, since stat-grafting means that Pikachu's own movepool is sufficient to stand alone without anyone needing to transform at any point.
Third, Custap Berry isn't currently obtainable in G7 games by any legal means, so the Quick Claw is necessary in its place.
[EDIT] After some in-game tests, I can verify that Power Split does take advantage of what was the defense stat after using Power Trick, so the 613 plan works. There you go: 645,647,940.