I’d say step one should def be make sure timing is correct, you’re getting wot, etc....... the basics.

Pull all plugs and do a cranking compression test on all cyls.

If it all checks out good, head to the chassis dyno for a few pulls.

When it had that valve float at 5500, it had a different intake on it, correct?
What was the speed then?

I’ll just throw this out there as another possibility ........ the converter might be going away.