I assume you want to reuse all the modifications possible. If you were to build a 351 with those parts, it wouldnt really be worth the effort. But you would see some gains obviously...IMO it would not be worth it
It's worth the build. You can use most of the parts in your sig except the lower intake, u can use the upper with a lightning lower. You will need to either get a roller block or convert an older block to roller which can get expensive.
You can use these parts to get it in the car and running but you will need better heads and a bigger cam designed for a 351 to make a real performer. Not to mention that there are a few swap parts that will be needed. Do the build, you will never look back.
you would gain an extra 20 - 30 horse just on displacement alone if equally built, you would problably gain another 20-25 with the RPM's more if you ditched the E cam. You will also need a lower intake for the 351 to take that upper. I see no reason why a flat topped 351 with RPM heads and retaining that E cam would not give you 40 good horsepower and about 40 extra pounds.