An old hotrodder once said "There's no replacement for displacement!" If you must run a small block in such a large car, the 400 will make the most torque and is therefore the logical choice. Eagle makes a "412" kit that is just great. I wouldn't bore it more than .030". Increasing the stroke a little won't hurt. Be certain it's honed with plates and the guy knows what he's doing. 400 bores are pretty thin and can be an adventure to get "round and straight". 3.75" strokes "like" 6" rods. Avoid cast crankshafts except genuine Chevrolet.
Your cooling system is paramount. 400s generate a lot of heat and it needs to be removed quickly. An external oil cooler helps, too. Don't scrimp on the radiator et al... Good fans, etc.
You can easily make 400-450 HP AND lb. ft. with a 400 with a reasonable idle quality, vacuum and low-end response, all necessary for a good performing street engine, especially in a 3,600 lb. car.