Go 5.4. Better rod ratio and more potential. MMR does sell a cast iron BB 5.4. Total cubic inches is 351. But its hella expensive. I think 3300$. And hasn't been proven that the sleeves don't shift. However, you can get the aluminum GT block that is a 5.4. Ofcourse that one is about the same price
($3200).

Bottom line to save your money and keep reliability relatively high, either for a FI motor keep stock displacement or a max effort N/A motor go BB.

-Mark