You seems like very new to MX6 and ask for a very old impractical idea.
I4 for V6 will cost you a lot of headaches and trouble as you basicly need to swap the whole car except the chassis or the body.
In the US, getting a V6 is just from hundreds to $1500. It is better to sell you I4 and buy an old V6 for couple hundred then swap KLZE. Trust me, this will cost you way less than swapping I4 to V6.
There is a thread in this site and I guess only i person actually did it. However, it was done long logn time ago when the value of V6 was still around $5000.
Hope this help
My Rides: 08 750I, 01 740IL, 99 Z3, 93 MX6 GREEN, 96 3000GT VR4, 99 3000GT, 03 Eclipse GT
My Wife: 03 ML 350, 08 Kia Sedona