Leon,
I agree with Captkaos on towing 3000 lbs in OD. I hope you have transmission cooler installed too. Heat will beat your OD transmission to an early death.
I have a little experience with using a carb spacer. There are several forms on the market. My truck currently has a four swirl hole 1 inch adaptor for a square bore 600 CFM Edelbrock. After the installation, I did notice better low end torque on my truck. Cruising in OD from 35MPH to 45 MPH in the city does not bog down my 305 as before the installation. Try the installation. It works for most users.
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Gerardo
1983 Custom Truck
TX Plates: "BAD BOW"
http://www.geocities.com/abetterchemist/