Struggling at high rpm above 6000

JBurch said

Here's a couple of wild guess's.........you are running out of gas, need more volume to the carb or valve springs are weak

Click to expand...

My first thought was valve float.
And the OP needs a 383. :poke:
Edit:
Now is a great time to switch to a solid flat tappet cam, conical springs tool steel retainers, light weight edm lifters, light weight pushrods and ductile iron rockers. Lighten up the entire valvetrain and reduce the number of moving parts. Solid vs hydraulic, flat tappet vs roller, ductile iron vs roller tip.