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You would probably be better off putting a 4.8, 5.3 or 6.0 vortec engine in. More power and easy to find. You'll still need a wiring harness, computer, fuel pump and regulator, also a proper flex plate and spacer for the th400 to mount to the block. There''s other options such as electric fans, drive by cable or wire (electric gas petal) and having your computer configured to what transmission you choose. You could also put a carburated 350 in as well, but you asked about fuel injection.
I'm not trying to discourage you and think its a great idea. There's a few things involved in installing fuel injection in a bombardier. I've done it and it cost me quite a bit of money but love the result.