Doing conversions is always going to involve some degree of hassle. I've done conversions such as a V8 in to a 6 cylinder Falcon, small block Chev into an HQ Holden, EJ20 turbo into a 1.8 Impreza, plus gearbox and rear end conversions. It is never as simple as you think, although it obviously helps if the car was designed for the bits going into it. A friend put a Toyota Supra engine into his Volvo 244 some time ago- and is still having issues with it.
The H6 is only 30mm or so longer than the H4, and uses chain drive for the cams instead of belts. The 3.0 litre version apparently is not big on torque- at least not down low. However, if you were to turbo it- then that sounds like a bit of fun if it worked out well. But seriously, being a pioneer in this field is never fun and most people who try it end up selling the car in frustration and loose heaps of money.
As for insurance you might want to see how the insurance company would view such a thing and compare that to the cost of an XT, which would outperform the H6 anyway. If you did go ahead with it then it would be good to see how it turned out.