I have a 455 in the GTO and a 400 in the T/A. Both are at different ends of the spectrum in terms of buildup. The 455 is lumpy and runs high compression #64 heads. The 400 is smooth and runs low compression heads.
The 455 definitely provides more torque, which is what moves ya. It also drinks more fuel and has different manners entirely. It drinks premium only and requires an exact tuneup.
The 400 is no slouch on torque, it just takes a lot longer to come on, not the slam of "Oh Crap!" power the 455 has. The 400 idles smoothly, gets excellent mileage, and is more behaved and appropriate for daily driving. It runs on Regular and is happy with almost any tuneup.. truly a dependable engine.
If fuel $ is no concern, you don't mind the lopey idle.. definately build a 455. If economy and balance are more your speed, try a nicely built 400.
You can build either engine as mild, or as radical as you wish. However, if you go with a 455, you're starting with a large displacement advantage.
Personally, I feel you can build a 455 a little more mildly to achieve the same torque goal as a 400 simply due to the mechanical advantage of the extra cubes.
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"I work in high speed aluminum tubing."
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