Well, Id agree to stick with the XR280 cam. Its a real close match to your compression and its a nice size in a 350(actually more like 360 with your overbore). I wouldnt want to stretch it much further than that if you intend on fairly regular street driving, cause it may get a little old.
As far as the manifold, you could really go either way Id say. Either the rpm you got now or the victor junior(not super victor) would be a good match. With that in mind(that yours is good), Id definitely stick with yours for now so that you have an accurate back to back comparison of what happened with the cam change , but no other changes at the time. However, Id still beleive that the victor junior will show a gain over 5500 or so, so keep that in mind if you want a little better number at the track down the road.
The vic will probably give up some bottom end compared to a dual plane....but, based on the drivetrain specs in your signature, the bottom end losses that you ACTUALLY NOTICE will be minimal, if any. With that converter you got and the 4.11 gears, you wont be relying on power a whole lot below 3500rpm, and below that rpm is where the single plane manifold should be "softest" compared to the dual plane. Thats why youd probably never feel it much(cause you wont be runnin there). From right around 3500rpm and up, it should be about equal to the dual plane, and it should start to come alive a little better around 5500(give or take a little)
Back to what I was sayin, just swap the cam to see for sure what just the cam change does for you. And, if you want to go faster, I do beleive that the victor would help out a bit for bracket racing. But if you go straght to the victor junior, you will never know for sure just how much that cam swap helped cause youd be making other changes at the same time.