An adjustable pushrod is for figuring out pushrod length only, and is a great help when working out the rocker geometry (especially with wider stroker engines). Were you using the rectangular shims that go under the rocker stands? If so then you've probably got the geometry in the ballpark, and the elephant's foot adjusters make pushrod length somewhat "forgiving". Did you shim the side play of the rockers as well (with the round shims that go on the shafts)?
The merged Sidewinder is exactly what you want; make sure to use the drop boxes so it gets tucked up higher for a Speedster, and you can get dual tips as well. Al
PS- 1 more question- are you sure you're getting full throttle at the gas pedal? Have someone hold the gas pedal down while you check at linkage on the engine. You wouldn't be the first to be tripped up by this...