The new A30 is faster than the old single carriageway which still runs alongside in places, but it's still a pretty fast road.
The route is a fair bit shorter than it was in 1966 because of new bridges, especially in Scotland.
That stretch from Warington is a PITA. When I did a ride of a similar nature to yours I timed it so that I'd do that bit at night to avoid traffic jams. I was at Warrington at arond midnight and a truckstop north of Preston for breakfast. It surprises me that record attempts don't use a similar plan. Hoppo doesn't sem to do what I did either when he's been going for his LE-JOG-LE. If do this section in the early hours and you have a support car you could probably send it up the road a little way to trigger off the traffic lights to get a green run too!
If you don't know, there is also a way to the Forth Road Bridge which avoids Edinburgh and is also a few miles shorter than going via Edinburgh. Easily found by using googlemaps route planner on "walking" from Carlisle to FRB.
Records are a lot about circumstance and all the stars coming into alignment. The 1966 record was done by two of the very best of their time with everything going right. Having a capable rider is just a part of it. Harder still when you need two capable riders. Then it's down to planning and a bit of luck.
FWIW, Wilko had a fair bit of headwind on his 1991 LE-JOG record, which he only just got.