Slow 6 more power

If by "power", you really mean "torque", i.e. when leaving a stop sign, the engine itself should be fine since they were used in pickups and vans. But, the gearing matters too. Later cars came with a taller rear-end for better mileage I think. There are some things you can do by swapping parts, like perhaps the later A-999 transmission, though not easy, and you probably don't want to give up good highway manners. Cheapest re-gearing is to just install smaller diameter rear tires, and if you don't change the speedometer pickup gear to match, you will at least think you are going faster. If a very noticeable problem, you probably just need to tune your carburetor and consider modernizing your spark (many cheap solutions).

Max power comes at high rpm, with the engine screaming. If that is what you mean, the easiest fix is to just floor it and insure the kick-down linkage works and is set correctly. You can even downshift by hand. My 1996 4 cyl minivan is like that. It has almost the same power as the V-6, thanks to its DOHC and 4 valves/cyl for great breathing at high rpm. But, it doesn't have near the low-end torque. To get the power, you can't be afraid to rev the engine. But, the slant is poor on high rpm breathing, mainly because the intake and exhaust are on the same side and it has small valves. The best way to over-come that is with a compressor to force air in. Cheapest version is a turbo-charger. There is no store-bought solution, but many have done and many posts. See youtube for a guys running <10 sec in the 1/4 mile, which beats a factory HellCat. Another is a belt-driven supercharger, which gives immediate torque. Actually, the very cheapest is to connect a gas-powered leaf-blower to the intake, and people do that with many cars just for kicks.