power falls off quickly

What do you all think?
1966 el camino- 9to1 compression 283, 3:55 rear, 2500 converter, 350 trans.
283- .060 over,cast pistons,arp rod bolts, cast crank and stock rods, heads- 3 angle valve job, ,010 mill, don't know the casting number but they are the factory heads for the 220 horse 283. Cam- crane hyd/flat 266-210/210, .440/.440, holley street dominator, dual plane. 600 cfm edelbrock carb. distibutor is hei-petronix. Timing is 13 initial/ 34 degrees at 3000. Has small tube headers and a flowmaster header back kit. The 283 came with the car ( numbers matching) and i'm not looking for track performance, more for good street motor for cruising.
Power is good until about 4000 then begins to fall off dramatically. I think it may be the heads as they are probably small runners and i did not elect to throw larger valves. Otherwise, they had a good valve job.
Initially, I thought that this was a good match of parts for the small cubic inch but i expected a bit more power and a longer power curve.
Any input would be helpful. Thanks

yep its the heads without any work they quickly fall off from 4-5000 rpm.

very nice elkie !