If you have the chance to convert from 350 TBI to Vortec 454, that's what I'd do. Otherwise, I'd keep the 350 and/or look for another. I know 350 TBI are pigs. You don't give much details on your truck. Is it 4x4? 1/2 ton, 3/4 ton, Burb, PU
All these things matter when asking folks for opinions.
Personally, I prefer a peppy small block over a Peanut Port headed 454. Small Block are alot more common, easily found and easy and cheap to make fun to drive peppy and chirpy rides. Big Blocks are more uncommon, harder to find, more expensive, drink more fuel in MOST cases, but not all if you can discipline yourself. I'm not one that can do that. If I have the torque, I'm using that **** so there goes the fuel.
Also, is this your daily driver, a toy, your workhorse, your bad weather rig? What the purpose? Many factors to consider when making these choices.