When I build onto a factoy Y, I whittle back till the new pipe is a clean weld to the factory pipe. I do this by squeezing the new pipe to an oval and trimming the factory part back till they mate.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.