The Holley Sniper is probably the best deal for you. There are various models and different price points. I'd probably go with a Stealth version but they are a little bit more expensive. The Stealth has the same basic size and shape as a Holley double pumper so your existing throttle linkage should hook up okay. I highly recommend that you download the instructions from the Holley website before you buy anything. There are a lot of details that you have to work thru with the wiring and fuel plumbing so do your homework up front rather than later.

I've installed a bunch of Snipers over the past several years. Most of them work great right out of the box but sometimes they do have a problem so make sure you buy from Summit and keep all of your packaging and documentation so you can return it easily if there is an issue.