different 318 question

Your cam is likely running out before the bigger carb, intake, and exhaust. Not uncommon to see this happen when someone just changes the 'in' and 'out' parts. I think I would go for a 256 or 262 cam max like 273 mentions. Getting lift up will be quite a help and all the cams mentioned will get the breathing up and not require you to mod the retainers. Get a good set of springs, get the exhaust rotators out, and add of decent timing chain; the old one is stretched by now.

Have you checked the compression on the cylinders? Any smoking from the exhaust on hard acceleration? If so, then the rings/rings lands are probably worn and that will cause the top end to drop off.