I don't have any specific answers or even specific suggestions, but one concern is the 520 cubic inches! A motor that big shouldn't need a lot of gear or converter. But that also looks like a lot of weight for that engine to move. Considering the narrow tires, it may be difficult to hook up IF you can figure out why it appears to be down on power. In other words, it may just blow through the tires if he engine is tuned to potential anyway. Big block (440, Hemi, 427, etc.) Stock Eliminator cars run low 10's and even high 9's on 9" tires and Cal-Tracs. But they have limited engine size, cam, heads, compression, etc. and a lot of time is spent on shocks, suspension adjustments and weight distribution. Track prep also has a lot to do with it. The more power you make, the more critical it is for everything to be "just right".

Gears will be easier to narrow down. If it hooks, you'll need a Dana 60. But converters can be tricky. I run converters advertised around 4,400. But they actually flash to 5,100-5,400 rpm on my 360! (Autometer playback tach.) Converter manufacturer need to have an idea of around where the torque peak is. Even then, similar advertised converters can vary a lot.