OOC: It's Alsthom at the time, and didn't become Alstom until 1999, and was GEC Alsthom from 1989 until 1999. Just for your info. 🙂
As for the design, as a fast-freight hauler you have a very good base here, provided the Bo-Bo+Bo-Bo can provide the speeds and acceleration needed for the NEC. Moving to 25 kV power would be a huge, huge benefit to the locomotives on the NEC, but it wouldn't be possible until there were locomotives there for the system to be used - such an upgrade would force the retirement of the GG1, as its electrical components would be blown up by sudden application of 25 kV power, unless you rebuilt their power systems, which for 25+ year old locomotives which were starting to have issues with frame cracks is a waste of time and money. The E33/E44/E60 trio developed for the Virginian, PRR and Amtrak would get the freight jobs job done, but none of the above are capable of fast passenger train service, and the E60s had serious issues trying. The New Haven EP-5s could do the passenger jobs but had appalling reliability, so for 25 kV power you would need either to slow down passenger trains along the route (not advisable) or develop a new passenger unit.
Assuming you did that, a fleet of 6000NAs, E60s, E44s and perhaps E33s (older at the time, but certainly usable until at least the 1980s) would give you a formidable freight engine fleet. Since this locomotive is made in the 1960s its probably a silicon diode rectifier locomotive (Alsthom was good at this), and remember that the E33/E44 fleet was pretty much bombproof, so the 6000NA would be expected to maintain such reliability and would need to be designed and built accordingly. If you're operating these on the ex-N&W you'd probably want to assign the E44s and E60s there first, for their immense power and heavy weight is ideal for heavy freight trains such as the coal loads the N&W made such profits on, while the 6000NAs, owing to their design, would probably quickly take over the NEC's freight operations with the 25 kV power upgrade.