I have a 1985 Monte Carlo that had a v6 throttle body in it and I got a 454 v8 carb engine with a 400 tranny. can someone help me with where I need to start with the conversion and how hard this is?

First off would be motor mounts, transmission cross members, and drive shaft length.

Do you happen to have any idea how much this conversion will run me

Dont forget the upgraded suspension and brake system. All told? Its going to cost alot unless you do it ylurself, and then you would still be better off getting a Monte built with a factory 305 and go from there.

Sell the big block and put an LS1 in it, you'll be a lot happier with it in the long run, you won't have to modify the brakes and suspension to accommodate the heavy set up the only mechanical mods would be what JRK mentioned and will up the value of the car when you sell it with an injected engine.