There are other reliable ways to provide rocker shaft oiling on a undrilled magnum block besides drilling to factory style passages.
You want the spacers to run a 340 journal crank? Properly executed spacers shouldn’t be an issue for your hp level but I think all but the 3.31 crank are available with 360 journal size. I’d put the money you’d spend on custom spacers into a custom forged crank that fits.
