I hope you are installing the Holley Sniper2, and not the original one. The original one had a lot of issues, even admitted by Holley engineers. Sniper2 works magic. I just had mine installed and finished. It has been great thus far, and hasn't even been tuned yet. Goes in on Monday for that.
I had the same throttle issue instantly. Garret, the metal fabricator guru I have referred to a lot in past posts, welded an extension to the throttle arm and it fixed the issue instantly. Now car revs up smoothly. Prior to the fix, it was difficult not to start accelerating fast. Basically I couldn't drive slow in a parking lot. It just jerked forward and sped up. Couldn't ease the throttle; now I can.
Any and every question you would have can be answered by my mechanic, Scott, who did the installation. He has done several of these installations and pretty much did it with his eyes closed. PM me and I will give you his contact number.
Also, I bought a tachometer adapter from Jon Haas at Pantera Electronics, which you will need to get your original mechanical tach to work with the Holley Sniper.
Fuzz