Slant won't drive over 25mph without cutting out

This is a 225 with a Carter BBD, HEI ignition and NOS Mopar distributor.

Drove it to work and everything was perfectly fine. Went to drive it home and it would stumble and die once I'd hit about 25mph. Seems to drive fine under that speed. It also idles the way it should in neutral. I can rev it up in neutral and everything seems smooth but as soon as it's in gear and I get a load on it and get the RPMs up, the thing dies out.

I put a vacuum gauge on it and normally I'm sitting smooth at about 20hg but now the needle is flickering semi rapidly between 16 and 18hg. I can detect a small vacuum leak at the throttle shaft bushing. I had it rebushed a few years back and there's always a small vacuum leak there since the shaft has to spin inside the bushing so I can't quite tell if it's excessive or not. Putting my finger on the outside of the shaft/bushing and plugging it that way makes no difference on the vacuum gauge.

I also noticed that I was getting arcing between the coil posts and center coil wire every couple minutes. I just ordered a tune up kit to correct that.

Any idea what may be going on here? I've had vacuum leaks in the past where I was only at 15hg on the gauge and it would cause a high curb idle but it never caused this issue where it wouldn't go over 25mph.