If the heads are GT40x, they will be aluminum. It's pretty easy to look at the head and see if its aluminum or not, will also have a label on the front of it like the picture below. Theres nothing really to worry about with the 4 cylinder to 8 cylinder swap, the only significant part swapping is the rear end from a 7.5 to an 8.8. Even if the car does have a 7.5, thats something you wouldnt even notice until you got into drag racing. You would never tell the difference on street tires. I can see from the picture that it has a 2 row radiator from a v8 car.
If the heads are GT40x aluminum, that definitely puts the value of this car above your car seeing as how aluminum heads are one of the more distinguishing mods you can do to a 5.0. That would almost put it in the "too good to be true" range on him wanting to trade for you car though in my opinion.
The only real thing to decide is how easy to car is to drive in your opinion. does it start up easy? is the throttle smooth and responsive? things like that. if it does have aluminum heads, id say trade 100% because its easily worth more than your car. It's probably worth more than your car even if it doesnt. Unless you can find something thats clearly wrong with the car or if it runs like shit, it should be easy to resell for at least your prelude asking price.
