The plot thickens Congrats! Hell of an investment there in good ol' P5153843 (casting number)
Experience that may or may not help: I blacksmithed an off the shelf 1/2" internal pickup to clear the Molnar rods on mine with 4.50 stroke, it did clear, but I hated it and felt pretty bad about it, so I went external pickup using the existing hole in the front of the block.
That hole in the front of the block is pretty convenient to run a single external pickup.
For internal only/no exceptions I'd either go directly to 4&1/4 or look into the less common 4&3/8 stroke which also might be OK.
Internal pickup tube could have been fully fabricated, or smaller size used.
Re: Smaller size---You know how the pickup adapter bolts on with two 1/4-20s...Back in the olden days they shipped a 3/8" npt with a wedge block, maybe they still do and it'll work easier for you (worth a look because it would have an easier time clearing the rods than the 1/2" that I used, regardless of stroke or connecting rod type).
