To start with, I'm wondering why you want more duration? More duration = more rpm. By adding duration you will move the power band up, but you're limiting at 6400? It kinda goes against what your build is. I would go the opposite.
With going to solid roller you will pickup some power, even if all cam specs are the same. Simply through the less drag the roller setup creates. You'll also pickup power going solid because you won't have the heavy hydraulic lifters, and the hydraulic action "dampens" some of the lift.
The number one reason (in my books) to go solid roller. Is to get a higher lift with lower duration. IE Wider power band with better low end torque. You can get a roller cam with 230ish duration, but still has .550 or more lift. This creates a wider power band, while still creating great drive-ability and good low end torque and throttle response.
In your instance, since you mention you are limiting to 6400 rpm (A little short for that cam in my opinion, but if you don't wanna spin it then don't). I would go with something in the following area
Duration @50 - 230-240 - Go with a split pattern cam, something like Lunati 40200730
Voodoo Solid Roller Cam - Chrysler 273-360 261/267 - Lunati Power
Lift - .550-.575. This will put you around the area that your heads flow best (although flow testing them higher and seeing where they actually drop off would be nice).
LSA - I would actually go 108. This will bring the power band lower (where you keep it). Yes, it will peak earlier, but whats that matter? You're shifting before that anyways. A LSA of 108 would also make more peak torque. Lower in the band. A tighter LSA will affect idle/drive-ability. But since you are going with less duration, the trade off is there.
Something like that should give you an engine with tighter/raspy throttle response. More torque at lower rpm. Lower power band. With those gears and in a 471" will REALLY fry the tires. I think next in your budget will be some drag radials.