The 531 head and its ports are designed to work at high rpm. In a car application, they will have reduced torque, stock for stock, than a 530 head, for two reasons:
1. The intake ports flow more, but at the expense of velocity and low velocity is not good for lower rpm performance, efficiency or torque.
2. The combustion chamber has a larger volume so you would be lowering your compression ratio which is worse for torque(aka, what moves you) and efficiency.
https://ozvolvo.org/discussion/8971/530-531-chamber-size-and-static-cr-measurements
For the port flow and velocity, it's not really worth installing a 531 or 405 head on a car unless you are planning on running a camshaft with at least 12mm or so of lift, as that's when you start really getting the benefit of the more advanced, and larger intake port design.
For the combustion chamber size difference and the loss of static compression, you can shave material off the head mating surface to bring it back up to, and beyond the 530's chamber size and effective static compression. That's no big deal, but you still are missing out on flow velocity and torque on what definitely sounds like a street car.
You should pull a 530 at the junkyard and call it day.
This, unless you're looking for performance above 4500rpm or so and are planning on using a camshaft more aggressive than the V.
That's another option, but then I'd have to swap the B cam to that head and shim it before installing it. Just weighing out what I should do. The better flow and cam seem nice for ~$290.
Swapping the B cam from one head to the other is the least of your worries...
Now I'm thinking of getting a decent 530 and milling off 0.040", bad idea for NA with B cam?
Good idea. You may need to run premium fuel and you might want to get an adjustable cam gear to move the powerband around to your liking. I would probably want to run it 2-4 degrees advanced in a non-turbo, automatic equipped car if that is what you're driving.
If you have a B cam that’s properly shimmed in your current head, you can just swap over the cam and all shims to the new junkyard head.
That is false. You may get lucky, but the valve stem heights of one head are not the same on all heads, especially with the varying amounts of wear. Swap just the camshaft over, then measure your valve clearances. If things are outside of .014-.020", you might want to adjust them. If they're within that, you will be "ok" to leave it as is. You might be able to mix and match with your shims from both heads to get things a little more even across the board, without getting a shim kit. If you want a smoother idle and more torque off idle, set everything on the looser side of specs, around .017-.018"(or so). If you want a little more top end pull at the expense of some idle quality and emissions, you can set the clearances tighter, around .014" or so.
Here are some misc. threads related to non-turbo performance I have saved over time, but it's by no means all of them.
http://forums.turbobricks.com/showthread.php?t=274989
http://forums.turbobricks.com/showthread.php?t=348598
http://forums.turbobricks.com/showthread.php?t=333776
http://forums.turbobricks.com/showthread.php?t=220554
http://forums.turbobricks.com/showthread.php?t=209648
http://forums.turbobricks.com/showthread.php?t=337311
http://forums.turbobricks.com/showthread.php?t=110700
http://forums.turbobricks.com/showthread.php?t=290198
http://forums.turbobricks.com/showthread.php?t=275416
http://forums.turbobricks.com/showthread.php?t=277707
http://forums.turbobricks.com/showthread.php?t=275133
http://forums.turbobricks.com/showthread.php?t=272196
http://forums.turbobricks.com/showthread.php?t=337956
http://forums.turbobricks.com/showthread.php?t=350635
http://forums.turbobricks.com/showthread.php?t=342490
http://forums.turbobricks.com/showthread.php?t=342456
http://forums.turbobricks.com/showthread.php?t=342098
http://forums.turbobricks.com/showthread.php?t=341814
http://forums.turbobricks.com/showthread.php?t=349476
And more.