is the 3.9 liter and the 5.2(318) liter engines in 98 dodge dakotas the same? i have read they are just the 3.9 has two cylinders cut off, i want to swap to the v8 but i dont know if i need a different trans to do that  

nope no different, the 318 will bolt right in to the transmission (might have to change the tq converter though im not sure on that one, i had my transmission overhauled while i was doing the v8 swap on my 4x4 dakota)  there are only a few things that you have to consider..  first off make sure u get the Dakota 4x4 oil pan for the 318 the other oil pans will hit the front diff, depending on motor and truck,  also make sure u get a magnum 5.2 as the LA 318s don't have the right motor mount bosses in the block to bolt up (i made that mistake) and depending on your year its a tight fit (87-90s)  also there are some fuel injection/computer things your going to have to sort out.  (i cheated i got the M1 manifold and a carb and controlled the transmission manually) hope this helps..   its a pretty straightforward swap.  

thanks, i didnt know were to start,