Eric -
First and foremost, the IMPCO AA3-64 kit is a Thermoquad only adapter plate to be installed inside the carburetor. It is not used with any other carburetors and, as far as I can tell, there's not equivalent adapter for other carburetors. (i.e., the 300A can be installed on other carburetors without modifying the carburetor.)
Closer reading of the IMPCO Thermoquad installations instructions leads me to believe the AA3-64 closes off the Thermoquad's gasoline systems, turning everything into a single (propane only) fuel system. So, if you want a dual (gasoline and propane) fuel system, you need to replace the Thermoquad.
Also, general consensus seems to be replacing Thermoquads with something else, largely due problems with the phenolic (plastic) float bowl.
Here's an excellent thread, with lots of pictures, of someone installing a used 300A on a Quadrajet. Read it thoroughly and carefully examine the pictures.
To repeat ... see if you can get the air cleaner in the picture you posted. (1) It's already adapted/modified for mounting a 300A. (2) It's a snorkel air cleaner, which I prefer. Snorkel air cleaners make it easier to install a custom cold air intake, which improves economy and performance. (I have an aftermarket dual snorkel air cleaner for MLP and will be installing twin cold air intakes in the grill.)
If you buy a used carburetor, definitely plan on rebuilding it or having someone rebuild it. Current best rebuild price in interior Alaska is $100USD plus parts, from one of the best carburetor rebuilders/specialists. So, plan on adding $150-250USD to the total cost of a used carburetor. If you rebuild it yourself, plan on an extra $50-100USD for the rebuild kit. (Make sure you get a rebuild kit with newer types of the seals ... old kits still on the shelf may have older type seals that 'modern' fuels destroy quickly.). Rebuilding a used carburetor before installation will save you the hassle of pulling it back off to have it rebuilt in the near future.
You really need to contact IMPCO's technical support. Those of us on this thread are really 'shooting in the dark' in trying to help you. There's some general things we can advise you on, like changing carburetors but you need to go to the source (IMPCO's technicians) for accurate detailed information and guidance.
BTW -
IMPCO has a European headquarters in Delfgauw, The Netherlands.. Without looking at a map, I believe this is fairly close to you.
