I would say wes has given the best advice in this thread.
Why take a chance at ruining a turbo housing if you have a steel manifold that is easily cut and welded. People giving advice that Welding mild steel to cast is easy and wont crack are just telling you storys and haven't really done it themselves. So why take a chance. My advice would be to put a hole where wes showed. The smoother the path into a y will work very well.
4 litre
1/You can rev a stock 4litre
2/strong bottom end and good pistons if using ef/l.
3/au engine you can use ba rods and pistons.
4/ twenty minute cam swaps.
250
1/weak rods and cast pistons.
2/ not happy to rev.
3/ cant do easy cam swaps.
4/CAN USE EF CRANK TO HELP IT REV