410 LA small block will not rev past 6000

The dramatic drop off at 6k suggests valvetrain [ control ] or ign. Ign is 'external', so a lot easier to check out. Is the ign box getting the reqd voltage?
If you have another dist, I would try that. Are you running a rev limiter? If so, I would disconnect it; & I would try another ECU & coil, third time lucky.....

Valve train. I do not think you need another cam. 1.6 rockers require more spring than 1.5 rockers. You could try some solid lifters, lashed at 0.006-8" hot [ & make them permanent ] . Your current hyd lifters may be bleeding down [ oil aeration ] or pumping up. Unfortunately troubleshooting the valvetrain may entail spending some $$$ unless you are lucky enough to have parts laying around....