Well, there we go...
That's a 256 adv duration cam, and likely the quench is above .070 and compression is a bit high for the cam .
I'd estimate your compression to be near 9.4 or so, could even be more, and with a cam that short, you'd need to be at 9:1 in a truck, essentially with your tire, weight, gear combo make it about equal to a 2.87:1 geared car with street tires. There is quite a load on that 390 and likely it's fighting itself inside with loose quench.
It would be nice to take a good look at the build, heads off, and see what you really have. Which head gaskets, which pistons, how deep in the hole, what the chambers measure, but the behavior seems accurate
It really depends where you are in terms of static compression, but my gut tells me cheapest way out is a 4 degree retard of the cam to 110 ICL, (assuming the compression isn't above 9.4 or so) and then set the distributor up to match the build, to include vacuum advance, and you'd likely be better, have more torque and be easier on fuel. The cam is small enough to still have plenty of torque with 401 cid
