87 b2600 motor revs up fine but truck slow

[kickindog]

I just bought my 87 b2600 the factory carb has been pulled off a new carb I was told that it was a 2 barrel beleived to be off of a dodge but thats all i know about it. It is on a custom intake manifold which seems to be sealed fine but i dont know for sure. The motor revs up just fine but yet the truck is extremely slow like going down a hill with the pedal to the floor and the wind to your back to make 70mph driving the hills and curves of the ozarks that isnt working very well for me and it makes quite a few ppl mad when they get stuck behind me. while going highway speed the motor does fine on level ground but going up hills it feels week. the truck has 31's on it i know that makes a difference but is it making that big of a difference cause i really like them and do not want to take them off i have replaced the fuel filter and i dont know if it matters as i am not a mechanic but im pretty sure its burning oil since i changed the oil and filter and like 2 weeks later what was left in it was black. the oil pressure reads around 60 pounds at temp and idling regularly. Also I think that its unrelated but im still curious the heater does not get very hot iv been told that this could be do to the lack of a shroud around the radiator fan and/or the heater core could be plugged up also it smells like antifreeze in the truck when its on. other than that its a great truck iv already put about 2k miles on it with no other problems at all. iv heard about v6 conversions but cant seem to find out what type of motor would work. any help is greatly appreciated.