Alright guys I need some answers, or guesses, as to what is happening with my new motor. I know its a fuel problem, because the car will idle fine, and it will rev fine, but as soon as I start going uphill the motor cuts out. I have not been able to get it to level ground to see if I have a similar problem, because its a hill either way out of my driveway, but she drives around in my driveway just fine. Now to give a little background so maybe its a little easier for someone to help me out. The car was originally a 2.8 V6, and has since been swapped out for a 400 SBC. I have not touched the tank so the sending unit and all is still original to the V6. I still have an in line V6 fuel filter, which I will be removing seeing as I might be getting vapor lock I think from it. All the factory fuel line is in place up to the frame rail in the engine bay, and the return line and evap line are capped off. I have I believe but I could be wrong 5/16" hard line from the driver side to the passenger side going along the rad, and then flex line going to my edelbrock mechanical fuel pump, and from there flex line up to my 750 speed demon. Now can anybody point me in the right direction to fixing this problem so I can finish my car and enjoy it the way it should be enjoyed? Any other information or pics that are needed just let me know, I need to find the problem.

A few basic tests would be in order.
A fuel pressure gauge to check pump output pressure.
Double check to be sure the carb float levels are adjusted correctly.
I understand that leaving the original fuel pump in the tank ( I assume it was previously fuel injected) can cause fuel pick up problems. Perhaps even prevent the mechanical from getting it's "gravity prime". If you remove the feed line from the pump ( tank to pump) the fuel should siphon easily from the tank. ( I suspect this may be your problem).
Check the fuel filter as you suggest. A clogged filter may only allow enough fuel to pass to maintain an idle.

I understand that leaving the original fuel pump in the tank ( I assume it was previously fuel injected) can cause fuel pick up problems. Perhaps even prevent the mechanical from getting it's "gravity prime". If you remove the feed line from the pump ( tank to pump) the fuel should siphon easily from the tank. ( I suspect this may be your problem).

Good call skinnyZ. Also does the tank vent properly?

I def. need to use a fuel pressure gauge to see where I stand on fuel pressure.
Carb floats are adjusted fine, its a 750 cfm Speed Demon Double Pumper.
Yes it was a 2.8 fuel injected car. I believe the pump is in-tank, so I need to drop the tank to pull that feed line off?
Going to remove the fuel filter when I have time over the next few days.

I have the cap not twisted all the way, because I was getting some vapor lock. When the car would stall it wouldnt start right back up. Take the cap off cuz we thought it ran out of gas, and the pressure releases and with a few cranks it starts. So now the gas cap is not twisted fully on, so now when the car stalls going uphill we drift to the top lol and after a few cranks it starts so I guess I solved my vapor lock problems. Just need a vented cap to really fix it I suppose.

Also, will removing the mechanical pump and going with an electric fuel pump and regulator help at all or will it do the same thing regardless?

There are plenty of electric pump, carb combinations out there. One advantage you have is that there is already a return line in place. This forum is full of both in tank and external pump installations. The 86 Firebird I work on has an in line pump near the tank (near the rear wheel house) and a regulator mounted on the front strut tower support. We keep a coarse filter before the pump and a fine filter after the regulator.
In my case, (86 IROC) I went from TPI to carb. I replaced the in tank pump and sending unit with one from a carbed application that has a pick up tube and sending unit only. I've kept the filter in the stock location however I find that to be inconvenient and will eventually relocate it to a location near the mechanical pump.

if the stock pumps works you could use a mallory 4309 return regulator and leave the pump in the tank...I did this for a tbi-carb and it works like a champ. just a thought good luck

How is your float level set?

I don't think this will help.

I think its float level/carb vent issue as well. You say you can't get to level ground - how did you set the float level? If its like a Holley, it will have a powervalve in the carb too - which one do you have in there? The wrong one could let it either flood the engine enough to bog it down and possibly kill it, or not let it open when it should and lean it out enough to kill it.