Ok, I'll throw a few thoughts out for you to consider, you might not like what you hear, but you will be happier with the car in the long run.

First off, throw horsepower numbers completely out of the equation..... in your case chances are it will never be on a dyno to find out and it's really only good for bragging rights anyway. You have a car that weighs around 4000 pounds if not a bit more (don't believe me, go weigh it). What you need to concentrate on is building torque. and a lot of it. Fortunalty you have decent engine for that purpose.

First off with an 80K motor the first thing to do is figure out what your going to do first. If you plan on driving it for a while then a few things will need to be looked at due to that age and mileage, and these things can be done with the engine in car.

First if the car does not have an oil gauge, add one. You want to see what the oil pressure is both driving and on a hot idle. If this checks out your next step will be a compression test to see what condition the top end of the engine is in. If these both check out you can likely start with bold on parts that will make the car happier.

At 80K you will want to pull the valve covers and replace the valve seals and the timing cover to replace the timing chain. While your doing this if you find that the valve seals are gone, or the timing chain is missing teeth (nylon gear)......pull the pan (always a fun job...check the shop manual for instructions). What will happen if you don't is that the pieces will eventually plug the oil pickup screen and at some pint will cause the little spring loaded trap door to open allowing pieces into the pump jamming it, twisting the oil pump driveshaft off and causing loss of oil pressure.

If it needs rebuilt, pull the engine go thru it, use quality parts, minimum overbore possible, and turn/polish the crank, keep compression at the 9:1 range or 9 3/4:1 range with aluminum heads. Add the parts listed below. If the engine is sound do the steps below, preferably in the order listed

If you have a sound engine to start with first thing is find out what gears you have in the rear end (there will be a small metal tag attached to one of the rear wend bolts that will list the ratio). At the expense of gas mileage you will probably want about a 3.50 gear to pep the car up a bit. A gear change and will run about $250 in parts, add another $250 if you want a Posi. With your background, get a manual and try setting them up yourself....otherwise $3-500 labor.

You do not have a C6 if it is the stock trans. The C6 were an all aluminum case and I believe the introduction year was 67. I think you will find you have an FMX (or possibly a cruso-matic) both have a cast-iron center section. The FMX is a decent trans, the only problem is that they are not supported well by the aftermarket for things like heavy duty parts and converters.

The old 390s had terribly restrictive exhaust manifolds. Your first step for the engine should be to replace these with headers either aftermarket of if you get lucky a set of cast-iron factory headers. Follow this with a 2 1/2 dual exhaust with a crossover pipe and low restriction mufflers. Your looking $1000-1500 (headers for FEs are high).

The old 390s like a BIT more cam go for something that makes power in the 1200-4500 RPM range RPM range (where you'll be operating almost). It would be a good time to do this when you are doing a new timing chain. Add a new set of valve spring while you at it. $100-200 range.

The factory intake is not all that bad (I'm assuming it is already a 4 BBL.....if it's not chances are you DO NOT have a 390, but a 352), but as mentioned it's heavy. If you elect to keep it, go with either a 650 or 750 (I know I'll hear screaming about the 750 but I think you will find the car a bit more responsive again at the expense of mileage). Carb cost are in the 250-300 range for Edelbrocks.

If you go with an aftermarket intake (you'll pick up a few HP and save a bunch of weight that you will appreciate if you ever have to pull it in car) make sure it in the same RPM range as the cam. Something like an Edelbrock Performer NOT the Performer RPM. About $200-250.

Add electronic ignition, one of the best would be as far as supportability and parts availability would be the 70's Ford Dura Spark unit. With rebuilt distributor, control box and wiring, your looking at around $200.

That should give you a start.