Don't be too discouraged,
You have allot of factors against you, 5800 ft above sea level/track "with good air", a little cloud cover, a decent prepped track surface, with out any corrected air density/altitude, it could have been more like 7000 ft when you ran, especially if it was in direct sunlight, in the heat of the day, with out much cloud cover...
You have a heavy 4200lb car on street tires, with "maybe" 375-400hp @ sea level not 5800 ft...
You have stock 906 cylinder heads, they are good for a stockish engine {Mopar Wedge heads are notorious for being the limiting factor, in making as much power as possible}, but they are a choke point for making better power levels, a good multi-angle performance valve job & some pocket porting would really help to wake up the 383 with 906 casting heads...
The Edelbrock Performer Intake is a smidgen better than a stock heavy cast iron 4bbl manifold, not really intended fro making higher RPM power, good to about 5500 rpm... not really intended on being a racing manifold... just an upgrade to the heavy stock cast iron manifold...
Your camshaft with stock heads, isn't able to work to it's full benefit, with out some head work/proper type valve job & the proper valve springs to match your camshaft, proper retainers & locks {lighter preferably} & along with the proper length push rods {often overlooked}, especially if you don't have adjustable rocker, to help you get the proper lifter preload....
The EFI might be helping some if it working correctly, keeping your mixture lean enough or rich @ those altitudes/conditions...
At higher altitudes you can "usually" run a little more initial & over all advanced timing, altitude likes compression & timing advance, your engine is choking trying to breath...
You have a manual transmission & a clutch, if your not proficient with your launch & shifts @ the proper specific RPM's, that can & will kill ET, very quickly & drastically, there's a reason why most people run Autos with a higher stall speed converter & a manual valve body drag racing, because it takes allot of the drivers mistakes out of the equations, for quicker & especially more consistent ET's...
Your clutch "could be limiting you some", if it's slipping, the more power you make & more traction you get the better clutch application you'll need, with a better tire, you would/should run a better clutch/with more application pressures, along with lighter forged or billet flywheel, for quicker RPM's & better throttle responses...
Steeper gears like a 4.10, 4.30, 4.56:1 would help some too for track use, smaller engines like more gearing to get over the fact your trying to move a heavy car out of the hole quickly, Steeper gears aid in the torque multiplications to get the car moving easier/quicker...
The 3.91:1 gears are a good compromise gear, so it can still be street driven with out too high of RPM @ highway speeds, sucking up fuel, but with a larger/more powerful, that engine that gear ratio would be more ideal...
At higher altitude all these factor, have to be as close as possible to perfect, to help you get the most out of your engine @ sea level there's a little more room for error...
Good luck in the future, practice & tuning, will get your #'s better,
I hope you still had fun,
I see many people that get discouraged the 1st time out, most usually have an unrealistic high expectation of how fast their car will be...