355 vs. 406 w/vortec heads

I have my 355 apart and it needs a few more things than I had planned. Actually, it only needs one piston(skirt has a crack) and all the basic overhaul items. No big deal. I was just about to pull the trigger on all the needed parts, But then over the weekend I went to a garage sale at a neighbors house and saw he was selling a complete 400 core for $100. Supposedly he drove it home, but it smoked. I started thinking that if this 400 core was in fact useable I could gain 51 cubes with just some new pistons and the typical block machine work + balancing... the other stuff i need to buy anyways. The engine is used in my K30 truck that is mud raced and sometimes street driven. i could always use more torque. My new cam set-up is probably gonna be rpm limited by the lifters, so I am thinking the larger displacement may really help me. Just looking for some opinions on rather the potential gain is worth the cost. I have no experience with the larger small blocks. I dont have alot of money, but i could make this happen if its worth it.

my current combo is:

10.68 CR @ .038 quench (was planning to mill for 11.2)
062's w/stock valves bowl blended to 88% throat.
RPM air gap
750 DP holley
1.75 headers/3" pipes
10" converter (3400 stall)
street hyd roller lifters (recommended for 6200-6500 max)
new custom cam is 240/244 @ .05 108lca .535" w/ 1.6 rockers std. base circle. IVC 74 abdc.

went to a hydraulic roller for reliability, and i really hate setting the lash on a lifted truck. I already had appropriate springs.

old cam was solid tappet, 248/256 @ .05 106lca .550/.572"(net)