Doing a few calcs and it looks like you need around another 60fwhp to go 5.90's, current 618>678, your 60's are good so not a great deal to be done there. As you said send them to Modern and see if you can get them to the 345cfm mark with some intake mods, not sure what they flow ootb though, 310?.....should be enough.

Different set up, but I went from 11.2@118.6 with stock 906's to 10.7@125 in the 1/4 with just a pair of BV 906's around 260cfm bolted on in a 3300lb 440ci car with a .650 solid cam.