Re: 351 Cleveland sluggish and breaks up at 5000 rpm
Lot's of stuff here and the exhaust is SURE not helping, likely to the tune of 50 hp if that is a rowdy engine
Here is how I would attack
1 - Check timing and timing curve. Look for two things, A - What the curve is from idle to when it stops coming in. B - Does the timing mark jump around ever? A will tell you if the timing is late, B will tell you if you have an ignition problem
Some additional things, with the Unilite, make sure that after running for a while, you check voltage AT THE POSITIVE SIDE OF THE COIL, yelling because people want to check the red wire to the module, that side of the module can handle 14+ volts. However, as the field collapses in the coil, "coil fires" , anything over about 11.5 volts will backfeed the Unilite and it will start to fail. Generally, the stock Ford resistor wire is not sufficient. Don't worry about resisting the coil too much, a change in voltage to a coil input has very minor changes to the output. If you decide the Unilite is failing, there are significantly cheaper alternatives with Accel or Mallory points elimination kits, same module but 1/2 price, then you can either fix the feed voltage, or add an MSD box to get to a low voltage trigger
2 - If timing and ignition is behaving check for WOT without trying to hard with someone pushing on the pedal, not with you working the throttle under hood. (engine off) Often floorboard design, angle, etc, will keep from complete full throttle. Once there, you work the secondaries and see if they are binding on a gasket or not opening for some reason. An example, I put a customers Chevy on a local chassis dyno and we went from about 400 RWHP to 500 RWHP with nothing but a timing change and getting full throttle
3 - Depending on the profile of the cam, and VERY likely, you have some dead valve springs. Might be a good time to borrow or buy an on-the-car valve spring pressure tester. At the same time, measure valve lift so you can see what open and closed are. Valve springs don't last forever and the amount of time, even if they were brand new, could have some tired ones. Also, your guy is incorrect, although a broken valve spring will skip, a soft one starts losing control and is seen as a jagged HP curve. They don't dead skip, and in fact, I have seen old springs act like a rev limiter in some milder applications. That alone could be your issue
4 - After checking the other things, be sure the valves are lashed correctly. It will change behavior and too tight is far easier on parts than too loose (within reason, the valves have to close)
5 - Finally, a high-winding 400 will never really make power with that exhaust. It shouldn't really break up, but it sure will lose a lot of power.
70 Sportsroof, 427 FE/489 cid, TKO-600, 31 spline 4.10, A/C. modified Mass-flo EFI/reprogrammed A9L/CnC ported Victor.