377sbc

as far as the ls series heads go,different port angles,and port lengths effect airflow than just the port volume.cannot compare apples to oranges.  

i forgot to ask if you degreed the cam.so sorry.  

I also agree that the 377 should pull at some r.p.m point. However, have you reved it to that point. Many 377 with large ports require a 8000 to 9000 launch and a shift point near the 8500 r.p.m. range. A solid built motor should with stand this r.p.m.. My 14.5:1 360 sbc makes its peak trq @ 7400 r.p.m. and runs out to a 8400 r.p.m. shift point. This motor has a 230 intake runner.
The only other thing I could think of is fuel delivery. I know you said you checked it out, but a good lauch followed by a slower e.t. is typical of fuel stavation.
How far off is your m.p.h.? This should also be much slower.  

I've revved it to 7500, but it doesn't run any quicker than changing at 6000, which when driving by the seat of ur pants feels about right to change gears at.
In the burnout it revs crisp and clean to the rev limiter at 7700. But as soon as the tyres start to warm up it just pulls the engine down.
I've degreed the cam and is spot on.
Plugs are a dark tan color. I didn't think they looked lean, although it's a bit different reading them when running unleaded fuel. (Am running 98 octane plus octane booster) Can hold 9 psi through the finishline. 825 BG carb
I'm only running 1/8th mile tracks and it's only running 7.9 @ 84 mph.  

But how uch slower is your M.P.H.? vs. the 383ci motor.  

Mph is within 1 mph. ET is about 2 tenths slower for an 1/8th mile  

So your m.p.h. shows that the engine is making power. About the same as the 383. Your 2 tenths e.t. is your launch and or wheel spin on the line.

How much does your car weigh?

I am sure you tried this but I will throw it out there. Have you tried advancing your timing 1 to 2 degrees and check your m.p.h after that change.

Or jetting up 1 or 2 jet sizes and then check your m.p.h..

Just throwing out some things that are over looked. Sometime we over-look the obvious thinking it is a complicated problem.  Good luck and please post your findings. I am intrested in what you find.  

what about your air cleaner? is it large?
ok sounds dumb, but i notice my commuter bogs on hills when it's alittle dirty.
i'm trying to figure out how to put a bigger one in or stack'm but not enough clearance.

open headers or mufflers. check exuast back pressure, with a fuel pressure guage. just a thought.

what about actual head cfm flow?  

just a thought.

a duel band fuel air ratio oxygen sensors set up would tell you if your runnung lean.
look at lsxtv.com for their video advert from power tv on the f.a.s.t duel band  readers with data logger.
 You can replay your run in full to show you if and where your running lean or fat.

Also iv seen on some cam / spring set up that a fast  ramp rate on a cam lobe can react badly with some springs and not others.  Some can get a valve flutter not a float . at a certain rpm the valve flutters looses combustion and dosnt have time to correct itself before it time to back off..

but i would be going down the path of the fuel air ratios  first to rectify ignition or fuel  falling off somewhere.   running fat could be lose of spark  at a designated rpm. my old hei distributor did this!