Sputtering off idle under load...

[Cal69Chevy]

I have a 69 Chevy with a fairly new crate 350 with the original manifold and Qjet carb. Just resently it has started to sputter off idle under load. The engine idles great and in park I can rev it slow or fast and it sounds great.

I'm not a trained mechanic but I'm able to figure things out by asking questions and reading from manuals. My first thought was this has to be a vacuum advance problem. Then some other said it sounds like a fuel starve situation. What do you guys think it is and how would I go about testing to determine exactly what is causing this?

BTW, the carb ID is 7029207 with indicates that it was for a manual transmission. I have a TT400. The original owner said the carb was stock. What's the differenc between this carb and on made for an automatic?

Thanks in advance for any help.

Ken