I am feeling like your compression is on the low side.What is your target goal for power?
To me for you to help your situation as far as the cams concerned tighten the lobe separation to 115 with 4 degrees advance is a start to get you some more low end grunt.
I still feel like more info is needed l.e cam card/valve events,head flow,exhaust setup,intake,
gears,quench height,boost level,and maybe current dyno sheet.

 

I'd have user Patrick G. over on tech spec out a cam for you. He seems to have quite a following of very pleased customers.

 

The one I have for my 370 is a 223/229 114+4 with high .5xx lift (cant remember exactly) from CamMotion. It did very well in the truck N/A before I installed the KB but was a little weak below 1500 withthe converter locked. That low rpm lag is not a problem now with the KB though. I had this cam spec'ed when the truck was still going to be a DD so I didn't want to go too agressive. It ended up being a little hotter than I expected and feels like it makes power well into the upper 6K rpms.
With your 408 I presume you could use a bit larger. I think you current exhaust lobe is great (assuming XER lobes), just bring down your intake to ~230 but keep the lift, close the lsa another 2* and you should have a very nice cam for your KB blown heavy Chevy.

 

no experience but I've been told KB's like BIG splits in the cam for the most part.

that 8.5 to 1 compression should make it a real pig out of boost as well. My boost motors are both 9.5...

 

With that said, I really dont think your current single pattern is hurting you too bad as long as you have a well designed exhaust. However, if you are only running a 1 5/8" header with a single 3" catback, you may need a big split to crutch your exhaust.

 

Dynatech supermaxx headers and single corsa cat back. so yeah, like you said, lol.

 

Have you thought about redoing your exhaust instead of a new cam? Kooks is supposed to be coming out with a 1 7/8 header for us sometime this year. A set of those, a custom y-pipe and a 4" diesel catback should flow plenty enough for your setup.
I am currently running 1 3/4" Pacesetters with 3" collectors, 3 to 3.5" custom Y-pipe thru a 3.5" cutout squeezing back down to a 3" Magnaflow and tailpipe. I think I am going to have to change it to fit my 4L80 as it crosses over right at the new trans mount location.

 

I have dynatech supermaxx headers, random technologies cats, and Corsa touring cat back. i'm 99% sure the exhaust is a single 3". I just spent $3500 for that exhaust system so there is not a chance in hell i'm taking it off. I had a 4" diesel exhaust on there that I was going to use but W2W said it would be stupid loud with the blower so i had them take it off. it was a stock dmax system.

 

You spent $3500 on Dynatechs header and cat setup and a corsa exhaust? DAMN....

 

headers were $1000, Cats were $500, corsa was $1000, Then there was the $1000 in parts and labor to connect all of that together. There is no Y pipe for the HD trucks for those headers.