Engine sputtering at about 65

I've got a 71 Dart swinger with a small block 340 using as my driver. Took a lot to get it rolling and shifting correctly and it does fine up until about 65, as soon as I ask for any more out of the engine, she starts to sputter.

Carb is a 650, wondering if that might be a bit too small or if it could be the fuel pump. Not sure of the best way to start troubleshooting this.

Unsure of the heads or the cam as I purchased it as it sits and the previous owner is clueless about all of it.

Ideas?