Here are some links to sites showing flow rates
http://www.jason.fletcher.net/tech/f...a/aluminum.htm
http://www.racingheadservice.com/Inf...ts_New.asp#SBF
For performance your aluminium GT40 heads are probably about the minimum spec that you'll want for a 351, considering the flow number of heads like the 4V 351 clevos.
However I think that stroking your 351 to 408 puts your engine in new territory so some more flow would be preferable.
Notwitstanding this - if you're anything like me you're probably thinking that the whole stoking exercise is expensive enough as it is. I'd probably go with what I had to start and then look around for a deal on heads when time/budget/circumstances allowed - If I felt it was needed.
The other factor is the cam, if your going bigger on the cam will your existing heads cope - its likely that the 408 will be able to swallow a bit more cam.
It's easy enough to change the heads with the engine in the car but the cam could be a bit more of an effort.
The availability of high flowing heads means that port jobs are out (uneconomical) unless you have a very good reason or the ability to DIY.
Once you start toting it all up have a good look at some crate engines (maybe short motors too) and what you could recover from your own
Good luck with your mods and post some progress pics for us.
LoBelly