Whats Wrong My Heirloom 1970 C2500

[BabeintheHoods]

After my dad died about ten years ago, I got the family truck. This truck used to haul! It was legendary among my dad's workers, he just didn't know that. 1970 GMC C2500 (Pseudo) Longhorn.
CE234 1Z 63191 model CE26034.



http://67-72chevytrucks.com/gallery/...1_Door_Jam.jpg

Yes I spilled oil on the exhaust manifold!

But, a few years before he died, he took it to get rebuilt for a bargain, and got what he paid for. The block is not the 350 he went in with. Since then, it has laboured going up grades, even mellow ones. When you tromp the gas, no real reaction, or as my friends say, "No Jam".





First, I know fricking nothing about engines, or trucks, so use small words. What is the culprit? Is it the popcorn popper on the top (the oil bath filter)? What tests should I do to eliminate that as an issue? I assume this is the original air filter, but maybe it is wrong for the block? What should I be replacing it with?

Second, where did the block come from and what is it? Any pictures of what it should look like. Is this my cause? Looking behind the fan belts on the block is T 407 TZM, that's all I can see, I think that's the VIN.



I think this is the casting number? 346250 GM K3



Third, the snorkel on the oil bath looks like a hose should be coming into it from the bottom? Laugh at me and explain.