Couple more things. MAKE SURE THAT NEW CARB IS JETTED PROPERY AND NOT OVER-JETTED. If the carb, even though it fits properly, is actually for a different application, say maybe a truck, it could have a bigger jet and that would definitely cause the symptoms you are describing. I know they put these engines into trucks well into the eighties'. IT IS POSSIBLE WHOEVER INSTALLED THE REPLACEMENT CARB COULD NOT GET THE EXACT ORIGINAL REPLACEMENT AND INSTALLED A CARB FOR A DIFFERENT APPLICATION WHICH HAS FATTER JETTING.
As far as an aftermarket valve cover, I have not been able to find one which fits the integral one-piece cylinder head which was used in these engines starting in 1975. "One-piece" meaning the intake manifold and the cylinder head are cast as one piece and cannot be seperated like a "normal" cylinder head where the intake manifold bolts onto the cylinder head. The 250 in my '77 Chevelle has the integral one piece like yours but the 250 in my '74 Chevelle has the conventional two piece. The valve covers are not the same and the bolt pattern is totally different. On my '74 I have the chrome valve cover you see advertised in say PAW and a couple of the other aftermarket mail order houses. It is nice but will not fit the 250's starting in 1975 with the integral head. As a matter of fact it will usually say just that in the catalog - "will not fit integral head". If you need a valve cover gasket, and sooner or later you will want to change it, and go to the local parts store the counter person will look it up and of course make sure he gives you the one for the integral head, since like the valve cover, is different from the older style. TO MY KNOWLEDGE, NOBODY MAKES AN AFTERMARKET VALVE COVER FOR THE INTEGRAL HEAD BECAUSE IT IS NOT POPULAR. As a matter of fact, if you really get into your 250, you should replace the head with an earlier pre-'75 250 cylinder head with the bolt-on intake. Then you can change the intake manifold and experiment with aftermarkes bolt-ons if you like. I do not know why General Motors ever went with the "integral" one-piece design. Maybe it was cheaper to produce and I guess it did eliminate the intake manifold gasket.
Maybe it flows a bit better.
If you do locate an aftermarket valve cover for it please let us know.
Goodnight, Ken B