Ummmmmmm.... OBD3?!?!?! HAHAHAHAHAHAHA OK first of all, NEVER go that mechanic again... Hes a flake... That swap has got to be one of the easiest Ive ever heard..... I reccomend you just swap the heads, just use the z6 block, with his old y8 head and distributor, injectors alternator, everything, and call it a day.... no need to do a complete swap, just switch blocks, or if you must leave the z6 in, check all wiring, be sure to use the proper distributor, and just go over all wiring, and vaccum routing and check the timing etc... and if you stay with the z6, I reccomend you get a conversion harness (OBDII --> OBDI) and run a P28......