Of course the bigger the motor the more hp and torque. Do you have any ci limits?
Go as big as you can. Most sbc guys go with a 434.
If you don't you'll just end up building a bigger motor sooner or later anyway. That's what happened to me. Started with a 509, then 548 then to a 632. Should have built the 632 in the begining.
1- Do you have to pull vaccum??
2- What type of mud do you run, Deep or fast Track?
These things will get you faster without working on the motor.
1- Loose some weight on the truck.
2-Ditch the 350 and get a Power glide
3 -With those gears you're still running the low side. High side is faster.
Tell me about the suspension and your limits. (rules).