Assuming the basic long block isn't hurt or worn out, you can do better with a lot of tuning. Just bolting on a carb and changine jets wont cut it.

Yes as stated the cam is a poor choice. But it should do better. Is the cam timing off? Like a whole tooth? I just can see any big block not being able to run 45 mph in 4th unless something is way way off or the thing smokes like a barbecue.

A compression test (leakdown is better) verifying cam timing and some distributor and carb tuning can do wonders and is cheap.