I generally try to avoid this question, because many are very opinionated about the "best" carburetor; however, since you did ask:
First, we need more information. Do you have a squarebore intake manifold or a spreadbore intake manifold? As Shaker suggested, DO NOT use an adapter; match the carburetor to the type of intake.
If you do not already have an intake??? Normally, I would suggest a spreadbore carburetor for street use; however, if the truck is going to be used like a truck (heavy loads, pulling trailer, etc.) a squarebore setup will work better. If the truck will be used for standard duty (rather than heavy duty), I would lean toward the spreadbore.
Now, once you make the choice based on the desired function of the truck, you can think about brands and sizes.
If you opt for the squarebore route, I would suggest something in the 625 CFM range. The 625 has approximately 250 CFM on the primary side, which will give good air velocity for good driveability, and fair fuel economy. If heavy duty use, the 250 CFM primary provides better transistion to the secondary than the 150 or 200 CFM on the primary of the spreadbore.
If you opt for the spreadbore, like Joe H, I would suggest one of the 800 CFM units (although I differ from Joe on the year). Most spreadbore carbs are 750 or 800. You will probably never use the full amount, but the amounts differ on primary and secondary. The primary side of the 750 is approximately 150; while the primary side of the 800 is approximately 200. The 150 is too small to use on this engine in a ton and a half truck. The 200 will give excellent throttle response, and the best possible mileage.
As to brands: YOUR CHOICE (or the choice of your tuner). Personally, I had very bad luck with Holleys when I was a young man and not rebuilding carbs. 3, that's right, THREE cars I purchased new ALL had engine fires within 9000 miles of purchase due to leaky Holley carbs. I think you can understand why I do not recommend them. Others have had different experiences. I have had even worse results with the e-carbs, so do not recommend them either. I have never used a Demon, so no personal experience. I would suggest either Carter or Rochester. If you opt for the spreadbore, then either the Carter TQ part number 9800s or an original Chevrolet QJet from a mid to late 1970's 400 would be an excellent choice. If you opt for the squarebore, then Carter 9625s or 9635s (NOT 9635sa) would be an excellent choice for a carb with an electric choke. Carter 4759s would be an excellent choice for a carb with a manual choke.
These would be my suggestions based on 40 plus years in the carburetor business. However, there are other choices that others might make.
The most important part of this post is an attempt to have you consider the use YOU have for your truck. Hopefully, then you can come to a decision.
Jon.
