350hp and 400 ftlbs can be made with a mild 350. Use 10:1 cr, edelbrock performer heads (170cc intake, 64cc chambers), and a cam with 210 to 215 degrees of duration.
I have seen a 350 built with 190 cc AFR heads that made 345 hp and 400 ftlbs with 10:1 and a very mild compcam 252H (206/206 duration).
If you use a 400 crank you will also need to use a 400 flywheel, 400 balancer, and chamfered bearings.
You may also run into flywheel size issues. A 400 flywheel is 168 tooth and you might be using a 153. However, I have seen some bolt on counter weight that convert a neutral flywheel into a counter weighed flywheel (400 flywheel). also the 400 balancer maybe bigger than your current balancer which could throw off the belt alignment. Might be better just to get the 400 crank internally balanced so you can use the same flywheel, balancer, and starter.
Gm makes 383 crate engine with a roller cam that makes about 350hp and over 400ftlbs. roller cams make more power.