GMC Repair: Heads, horsepower calculator, roller rockers

Question
I have a 65 El Camino (First Car) that I've owned since 1992. The previous owner took out the 283 an installed a 350 from a 1972 Chevrolet Truck. He modified the 350 to give it more power with: Bigger intake, and a higher lift cam. I have no idea how big the cam is, but we had to put in bigger springs because I kept breaking them. The heads are 3998993 1.94/1.50 heads with the big 75cc chambers. Pretty much a low performance truck head. I can purchase a set of used double hump heads that a guy just took off his nova for about 200 bucks (He's going to a 383 stroker) Casting #3917291 64cc 1.94 1.50.....I only want to get these if I can simply have someone remove mine and install the others.  I've never had this done, so I was wondering if this is an easy swap, or am I opening a can of worms that will cost me big bucks. Also, what kind of HP do you think I would gain? Maybe 10-20? Obviously a higher compression.   I hope I haven't hit you with too much, and I greatly appreciate any insight you might give.

Answer
The first thing you can do with the El Camino is sell it to me. Is the intake a high rise? What brand and model? What carb? Distributor, plugs, wires? What springs? OEM springs? You will still have the same problem with breaking springs. I would get some good push rods, springs, and roller rockers. The heads are not a bad job, only time consuming. To save time, I will include a link for a horsepower calculator. You know more of the specifics. I would venture to say the HP would be more around 30, and could be in the mid range. If you run a high rise intake you will lose some torque. I would strongly urge the exchange of the springs, push rods, roller rockers. That will help too. And you can gain 10 HP but changing your oil to Royal Purple full synthetic.

http://www.race-cars.net/calculators/horsepower_calculator.html

http://www.cprparts.com/HPcalculator.html