Well, my first question (in return) is how do you figure the 350 has less low end than the 250 does? Low end is very significantly affected by displacement and the 350 has 40% more of that. A near stock 250 will almost certainly have less low end than a near stock 350. But you did say "right now" perhaps in reference to the 350 being in poor shape maybe? Not sure, but head to head normally a 350 has more bottom end. I just want you to know what you're getting in to.
If you do go turbo 250, I would recommend the 4 bbl. Overall a 4 bbl would be better for performance and I think offer more options for tuning. But I'm not a carb guy, so you'll want to back this up.
