To get the maximum power out of an engine, the density of the air-fuel mixture in the combustion chamber needs to be maximized. One way of doing this is to use a large carburetor with a small pressure drop at wide open throttle, as is commonly done with 4bbl gasoline carburetors. Impco-style propane mixers are more restrictive because of the nature of the gas valve that meters propane into the air flow. While it's possible to use an oversized mixer on smaller engines, the largest mixer generates 1.5" Hg of pressure drop at 460 CFM. The lower the manifold vacuum, the higher the fuel mixture density but using a oversized mixer runs the risk of poor low-speed operation and hard starting.
It is possible use a pair of mixers and Impco has a dual Model 425 mixer adapter to use with a single square-body throttle body (PN AA3-80) but this adds 5" of the height between the throttle body and mixers. Unfortunately, the air cleaner assembly for this setup is obsolete and you would have to fabricate one or find an aftermarket assembly. The AA3-80 adapter can also be fitted with adapters to use other mixers.
Megajoltman on the Moparts forum posted these photos of his dual OHG X-450 mixer setup mounted on a big block Chevy engine.
This is mono-fuel system using a pair of Model 425 mixers and a dual mixer adapter that the vehicle owner (Dodgen1) fabricated from a pair of AA2-46 adapters for his 440 powered 1975 Dodge W100 4X4. See My Toy for more information.