I am looking to replace my 351m and rebuild a 400m. How much power can I get out of a 400m and what parts do I need to get the most out of it that I can. I am looking for something good in mud.

Only thing needed to turn your 351 into a 400 is con rods, all other parts are the same including pistons. 4 bbl intake, bigger cam and a good 4bbl carb will really wake up your 351. Not much difference fuel wise between 351 and 400. Bigger displacement most always = more power/torque. I built a 351m about 20 yrs ago. 10.5 to 1 pistons, huge cam and 780 d/feed double pump holley with off road kit. Ran like a scalded dog. Miss the power, mileage wasn't a whole lot worse than stock.

Well I have a brand new edelbrock carb and intake to go on it, should I bore the cylinders out?

correction, you need a new crank and pistons to convert your 351 into a 400 the rods are the only things that are the same between both