Looking at the tilemap table in CustomBill.asm, it looks like RussianMan created an SP2 graphics file containing vertical bullet bill graphics. Specifically, I think you'd want to use YYCHR to create a copy of GFX01, replicate the bullet bill graphics, rotate it 90 degrees, and paste it immediately to the left of the original bullet bill.
This does overwrite some Koopa graphics, so if you'll be using Koopas in the same level as this, your best bet is to remap the graphics tilemap to a different location.
Also, I'm realizing that I don't know enough about ASM to say for sure if it's in the SP2 graphics slot, I think it could also technically be in SP4. I assume it's SP2 because that's where the original bullet bill lives, at 0x4A6.
Let me know if you need more help, I'll do my best to clarify.