400 sbc Power Delivery Issue

[Jak220]

So I have a 1984 K10 I've been working on for awhile now and lately I've had an issue of the truck being particularly sluggish, it doesn't want to respond much to the throttle at all.

Trans and engine have been swapped, has a 400sbc with a edelbrock aluminum manifold and heads, Rochester quadrajet, and HEI distributor. My uncle rebuilt it and I am trying to get some more info from him RE what cam he put in there, etc. but I know the motor is loaded out and it was only put together and put in the truck a few years ago. So please, no smart comments like "it's a 40 year old squarebody what did you expect, a Maserati?". I know how the truck is supposed to handle and there's definitely something up. The other thing is it doesn't feel very torquey at all, I know those small blocks are supposed to have a good amount of torque but that could have more to do with the gear ratio and tires so I'm not too concerned with that right now. Suspension is stock and tires aren't that big though so I don't think they're the primary contributors to this power issue.

It doesn't sound like it's running rough, no misfires, no stalling, starts up super easy even from a cold start and idles well. No issues shifting or slipping or anything like that and trans fluid looks great so I don't think it's a torque converter issue. I'm basically trying to run through a checklist of what could possibly be going on and just crossing them off one by one.

Here's what I got so far:

Make sure the choke's opening up all the way
Make sure the secondaries on the carb aren't seized/not under enough tension
Test for vacuum leak on carb manifold vacuum
Test for vacuum leak on intake manifold to make sure exhaust isn't obstructed (it's got the original tailpipes on it so this is definitely a possibility)
Check the wiring harness on the distributor under the cap (I know those pins can get pushed out and cause issues)
Check timing and make sure distributor timing advances properly at 3500 rpm (ensure weights/springs aren't seized)
Install fuel pressure gauge and make sure fuel pump/filter isn't gummed up

Any other recommendations for things I should check?
My experience so far is that usually when something like this happens it's a stupid simple fix, I'm definitely guilty of overcomplicating things 

Any advice/pointers would be greatly appreciated!

[JohnnyPopper]

I have to say, 'sluggish' is somewhat vague. At operational temp, what happens when you go WideOpenThrottle? Does she dump, as in choke?

When was the last time you did a tune up i.e. plugs, wires, rotor, cap, 65000v coil?

Have you checked the inline fuel filter at the carb?

[Shifty]

This is going to sound silly, but check the firing order as well.  I once bought my girlfriend a Camaro that allegedly had a brand-new GM crate motor, which ran fine, but had no horsepower at all.  The seller said that he wanted it gone, male any offer....I got it for a song, drove a few blocks away, re-arranged the wires, and left a nice pair of rubber tracks out of there.