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Nope. peak torque typically occurs at 3400 rpm. Your experience is typical. You do seem restricted at your top end if it falls off at 5000. Most midly improved 428's fall over at 5500-5600. The 1968 428 CJ makes 335 hp at 5500 rpm and 440 ft lbs of torque at 3400.
This could be a result of valve size, carb size intake or (not typically) exhaust pressure. To improve your top end your bottom end will suffer enless you bump compression to compensate. You may want to start with finding out what heads you have. heads with valves less than 2.09/1.66 are pretty much going to slow you down no matter which intake manifold and cam you have.
Heads are easily available from Edelbrock for $1200. Less expensive ones can be had on e-bay. I have a set of rebuilt 1968 GT heads I got on line for $550. After that I would say a stout cam. Not too much but not too little if you plan on driving it in traffic. 6000 rpm is not a great task any good performance grind will get you there.
Roller rocker arms will free up horsepressure but do nothing for your torque. They generally require an adjustable (solid lifter cam) arrangement which you may not really want or need.
Carb selection is another sore spot. If you are running an 850 (with a 5000 rpm redline) you have 150% of the carb you need. Right now a single Holley 600 cfm unit is too much for your engine. Even at 6000 rpm you only need just under 600 cfm. for a typical street engine with a VE of 85-88%.
Good Luck... Hope this helps,
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michael
A man that is young in years, may be old in hours, if he have lost no time. But that happeneth rarely. Generally, youth is like the first cogitations, not so wise as the second. For there is a youth in thoughts, as well as in ages... Sir Francis Bacon (1561-1626)
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