Hey Brad, thru-out the 70's and the 80's ford universal carburetor for that 460 was a 600 CFM.
That being said, it really depends upon what RPM you want to be running, how your engine has been modified and how efficient it is, and how much gas you want to be burning.
So lets look at those things, Odds are you are probably not spinning the hell out of your 460, and so 5000 rpm to 6500 rpm is somewhere in the range. If you are stock then it is probably closer to 5000 than the higher number. For that big trailer you are really wanting torque not horspower so your max torgue is probably happening around 4000 to 5500.
That 460 with its toilet bowl chambers is fairly efficient more so than a regular engine probably in the 70-85% range which is a whole lot better than 50-65% for most engines.
Per the formula, at 85% efficiency and spinning her 5000 your CFM requirements are 565CFM
at 5500 its 620, 6000 is 660, and 6500 is 730.
Willing to bet yours is not 85% efficient but that gives you a starting point.
From my experience and if you came to my shop, I would tell you if you ALREADY HAVE the 600 CFM carb and/or you wanted the autolite 4100, I would steer you that way.
If you were looking to buy a carb, I would steer you towards a cheap used 600 or 650 CFM holley whichever you could get cheapest, probably in the 5 to 20 dollar range. Preferably off of a FORD with a FORD number on the housing, and making sure the upmost consideration is its complete and intact and not damaged.
Either the 600 or 650 would provide for what you are wanting to do, give you some good pulling power and be easy on the gas.
I would take a look at definetly upgrading your exhaust system to duals, headers and very freeflowing, and then possibly an RV type cam with better valves if you had an extra 300-500 to spend. Would also have you looking around for a used aluminum intake, for increased flow and reduced weight but that stock 460 flows pretty damn good, at least as good as half of the ones on the market.
700 to 730 to 750CFM would give you lots of horses and your 460 would really like that but your wallet would not, nor would it be necessary in a realistic world. If you were wealthy and had no problem supporting the oil companies, crossing major mountain passes and then using your truck to haul large loads or just plane horsing around, or if your engine was recently rebuilt and a snarling beast then this is the way I would steer you. but odds are you would already have this combo on your vehicle.
Hope that helps,
Bill White
White Automotive
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HAVE A NICE DAY!!!!