Well, it's my turn to ask for suggestions..

Well, that would kinda depend on the size of the "rod",lol. A broomstick was mentioned. Ima thinkin the "rod" should be a minimum of 1.5 inches diameter, and I would like it to pass through the flex pipe all the way into the headpipes. I'm thinking a small plastic vacuum cleaner hose might do it, like on a "shopvac"
Hey if you are still running the factory manifolds you might want to prove the heat-riser valve is open. I know you said the previous engine ran fine, but it just takes a sec to check.
BTW, in my experience,an engine with valve float/lifter pump-up runs normal, right up to, when the valves fail to close. Then it flutters a bit, and goes flat, and more or less refuses to rev any higher.Then when you back out of it,because it takes time for the lifters to bleed back down, and they don't all bleed down at the same time, the engine runs quite rough,cuz the valves are not all closig properly. If you allow the Rs to come down too far, it may stall. It may not restart without a great deal of trouble or time, and will continue to run rough until the lifters are back to normal.This assumes the valve stems did not get bent during the event.
A plugged exhaust is a more gentle event, and the engine never runs rough after backing out of it. It just seems to reach a point when it doesn't want to rev any higher, and this point varies with load.It might rev progressively lower in each gear range.
-I should also mention, that a long long time ago, when I was really broke, I intentionally ran a teener with the cam advanced one tooth. The chain was so stretched, that performance was dropping. Nothing like yours, but I could feel it wasn't right, and the quick check revealed a well worn chain.I had previously installed a used 340 chain kit on her as I do on all my teeners. No, not all used; but,all roller chains.lol. So I simply moved the chain over one tooth, slapped it all together and reset the ignition timing. Voila, I was back in action.Did I mention I was broke? What I learned from that, was that there is a lot of leeway in cam timing on a small-cam engine.If I remember, the stock teener cam is under 200*@050. I seem to remember it being 192ish. That's a small cam.But it had to be, to keep the Dcr up, with that pitiful compression ratio.