I have the sp350/357 gm motor in my 77 k-5, (basically the same as the 350 h.o. but with a larger roller cam) and it was a bit lame on the bottom end out the gate. So I went to timing first, ( I am at 5,000 feet elevation) and the sp350 really liked a lot of initial timing. Its now set at 16 with another 6 degree's with the vacuum can on full vacuum at idle for a total of 22 degree's. I also changed the springs in the h.e.i. so I am all in by 2,800 rpm. I have the weights pinned so I get a total advance of 34 degree's, thats what my motor seems to like, (don't go by books , give the motor what it wants) You will have to play with your own timing because you are at sea level and have less cam, but you need to start there, (try getting your total advance all in by 3,000 rpm and set at a max of 34 and if it pings back it down from there). Then you can play with the initial timing with vacuum cans on full vacuum to get what your motor likes at idle. This is just one way of doing timing, (there are many), I just like to start with total and work backwards.
Then comes the carb, an afr meter in the exhaust is the way to go for tuning. If you don't have that you set the carb. by vacuum gauge for idle mixture, (highest reading) and thats usually a bit lean sometimes but go from there. Then with a holley if you have a bog I always mess with the pump squirters first , ( try bigger ) and if that is no good go smaller. The holley carb. thing is a lot to cover so you may want a book on tuning.
My sp350/357 likes a holley 650 double pumper the best. I also went to a higher stall converter for my trans. (2,000 rpm to 2,200), and that helped as well for low end.
Just keep playing with it.
