You should be able to turn around 6000 rpms briefly without too much worry..From the looks of your combo though your engine probably won't make anymore power above 5500 rpm so I would stick with that for a redline...A performer rpm or stealth intake would allow a little more useable rpm up top...As a rough example of 1/4 mile times my old setup was a 1982 F-100 with a 1969 429 thunderjet (320hp 2V version)..It was completely stock except for a set of headers and a 750 holley and stock cast iron 4V intake..It had close to 90,000 miles on it and it had the stock C-6 trans and an open 2.75 rear end...I ran the 1/4 in the 15.4 range@ a tick over 91 mph..I later discovered I had a severe vacumm leak under the carb so I'm sure it could have ran much better..The gearing obviously didn't help either as I could only use first and second gear..This was back in 1991 and I would run almost dead even with the stock 5.0 mustangs so it wasn't too bad for what it was..

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1968 mustang..408 windsor.Vic jr heads and intake.Pro Systems 950HP holley, 1 3/4 hooker super comps,Custom CI solid roller camshaft,Probe shaft rockers, 4 speed with 4:11 detroit locker.