My truck has a 6145, but I want to change it to a 1920. Is the manifold mounting the same for these?

Yes, but make sure you really know exactly what you're doing (not just someone said "Oh yeah, you should totally do that") before you go ahead. A 1920 is generally not a wise or productive swap onto a vehicle new enough to have come with a 6145; you can make better choices.

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Too many people who were born on third base actually believe they've hit a triple.

The only interest I have in it is that it’s simpler, and I can use the better looking air filter housing that fits the 1920. all traces of the computer, electronics and auto choke are eliminated. (I have a manual) 1920 is probably perfectly adequate performance wise. I never had issues with my 71 van that had one. What downsides are you aware of though? I could also put together a 1945 if need be. Thanks!