I've thought about this as well (a la Mechatronik). But I don't think the installation would be as straightforward, especially connecting up the electronics & interfaces to the computer (eg speed sensors, ABS, automatic, etc).
I would love to install the current 3.5litre V6 & seven-speed auto, but it doesn't seem like a straightforward installation to me, hence the sky-high prices for these conversions from Mechatronik & co.