Would a primary (front) metering block from a 750 vacuum secondary carb work on the front of an 850 DP? I bought an 850 DP off ebay and the front metering block has 3440 on it instead of 4781. I've got an old 3310 carb for parts and was wondering if this would work. Thanks.

Should work,however 4 screws and a fuel line will tell you real quick !

Probably the only difference will be the passages behind the power valve, you may have to slightly enlarge them for the 850 enrichment. Another thing, you stated an old 750, see if your carb has down leg boosters, you may have an old 780 carb (3310-1) and may be sought after by some.

You're right Godscountry, it does have the down leg boosters, so I guess it's a 780 carb. Good call, thanks for the heads up.

A 750 will have the straight boosters and have 3310-2, 3310-3, etc... The 780 will have 3310, 33101 or 3310-1.

What's wrong with trying the block stamped 3440?

The 3440 block is actually off a 2 barrel Holley, I think used on a Ford tri-power set up. The internal air bleeds will be in different locations and have very small power valve feeds. It will bolt on but you'd have a heck of a time getting it tuned to work on an 850.