Not based on rpm? Got an iphone make some videos and lets hear it.
If it does not get faster based on rpm its something out side the motor knocking against the block. But if it rattles when you let off the gas its a lose rod bolt. Under pressure they make a little less noise. Not much...
Check all the bolt on items. Alt, water pump, powersteering. Make sure its not something to do with the belts making all the noise. I would go pver everything before i ripped the engine out of the van. But i got a feeling its coming out. Maybe setup an engine stand to run the engine out of the truck and. Check it before/after to make sure its ready to install. Fyi. Sometimes its better to Use wallet and let a younger back go to work for you and remove the engine. If theoil pan is not clear then not much point in pulling it. You will need room to work.
I would drop the trans and start it back up and see if the noise is still there. But its probably got to come out and most vans are not going to make this easy unless you go with open exhuast. Then you wont be able to hear.
Never used summits blocks but did they give you rod bolts or did you buy your own. Are they all tighted so the flat side is facing the block not a point. 5.7 rods or 6.0? You didnt use stock rod bolt?
Did you Torque the rod bolts according to the spec of the bolts or the block? Also does your engine or bolts require tightening and losing the bolts during the install process? Rod resized during bolt change?
Sorry for the questions and no answers but trying to get a handle on the problem. Also some of them are just to lead you in the right direction. No need to answer everything with long details.