Well, your question really can't be answered very well. Primarily there are a lot of factors that are unanswered. To begin with everything should be replaced that deteriorates. Gaskets, hoses, etc. The bearings and rings probably should be replaced or at least checked. Soft plugs should be replaced. Next anything broken or showing excessive wear should be replaced. I.E. Pistons, etc. The bores should be checked, main bearing clearances, etc.
Basically the above amounts to the inspection stage. You are finding out what definately needs to be replaced. If for instance you find the cylinders need to be bored over then it will effect what needs to be replaced since you will need new pistons.
Next you need to decide what you will use it for. Then build it approprately. For normal use around town it can stay pretty much stock. If you will be giving it a hard time then consider performance parts life forged pistons and crank. You might consider adding a mild cam as well if you plan to push it on the street/track.
If you plan to use it offroad as this thread implies then you want to be careful in your cam choice. Most performance cams are meant for top end use. I.E. They light up at higher rpms and lob at an idle. While this souns neat in an off road truck it really isn't the best route unless perhaps you are playing in the mud. Instead you should try to lower the rpm range where the cam lights up if you will be running trails. When you are climbing a hill or going down the other side you want the engine running smoothly at very low rpms. Look at RV cams since they are geared more towards the low ends. Your goal is to get your torque ealier for stump pulling type power.
Carburetors are another issue altogether. Some just do not perform well offroad by design. For instance Holley carbs are very quick to dump fuel over the top and flood out. (I have a performer intake and Holley carb that are almost new if you really want one although I do not suggest it) Speaking of intakes your intake should also be set upo for the rpm range you want. The performer intake is a good choice for the low end torque if the right carb is on it. AFB carburetors (Edelbrock/Carter) are better than Holley and can have off road kits added that work pretty well. (I use to run this set up on my old Scout II with a Chrysler 413 in it) Quadrajet carburetors are supposed to be the best of the bunch and are said to work almost as well as fuel injection. I've never used one myself but that would be my choice if I were to use another carburetor. That brings me to the top choice in off road fuel delivery. That being Fuel Injection since it deosn't suffer the same off camber issues that carburetors do. (The reason I have the Performer intake and Holley carb available is because I chose this route) If you do consider this then my suggestion is to look over the information at many sites already for EFI swaps using later model Ford doner cars.
As you can see there are many options. I chose to go over a few of the options I think are important although this may not apply to you as you may use your Bronco differently. I do a lot of wheeling in the mountains where steep hills and gear busting rocks are common. (I busted a heavy duty D44 axle last time out) If you are using yours for trips to the store and will not get offroad much then you can get by with simply freshening up the motor and adding a mild rv cam. (To help get those big tires rolling) Then add a decent intake and carburetor. (Don't go overboard) Then look at changing to electronic igntion if it still has points. If you are going to run in the mud then look at raising the rpm range and using performance pars inside the engine that can handle higher rpms in addition to the above. (Cam, intake, carb for higher rpms) For serious off road use look at a mix of both. I.E. Performace parts but set it up for the lower rpm ranges. Also consider EFI or at least a carb that has proven itself in off road use. As to exact horsepower gains that is a term best left on the track. The amount of torque and the rpm range where it takes off will be a much greater concern. It does no good to have 400 HP at 4000 RPM if you are climbing up the side of a hill at 1 or 2 MPH. At that point you want lots of torque without racing the engine.
