I have Dodge Ram 1500 Conversion Van (1999) .
The engine is 3.9L V6.
The van itself did not have any major problem.
However, I would like have more power.

So, is it possible to replace the V6 engine with V8 engine? - maybe along with transmission

Or is there some easier way to get some more power out of this current V6 engine?

 

The best and cheapest way to swap to a V8 is to buy one cheap that has problems with stuff you don't need, like a lot of body rot or dents. Then you can swap all of the engine wiring, computer, brackets, engine, etc. over to yours.

The only other somewhat cheap way to get more power is to change your rearend gears to a lower ratio. This will of course cause your engine to rev a higher at highway speeds.

 

The question is what do you mean by power - low end torque or top speed? Changing the gears will help with low end torque(eg starting power). Changing the exaust and intake will help more air, and thus more fuel, flow though the system (slightly higher top speed). This thing is a fairly heavy van, and it has a large cross section of air to push out of the way. It wont be sprightly off the line no matter what you do with it, but you have comfort that anyone else has to stop driving to get.

Do you do mostly highway of mostly city driving? Do you know what the gearing is on your rear diff currently and which transmission you have? I think the differential gearing is on the door plate sticker, or if it is still there the vehicle option list under the hood on the drivers side.