Thats awesome you are building this car with your kid! I was 14 also when my dad gave me his 74 Duster...I still have it to this day, I will never get rid of it. The answer to your question If you go with the 318 motor you will probably want the 904 Transmission. If you compare the 904 to 727 there really isnt that much difference for what you are trying to do. My advice is to find a A body car with disc brakes being parted out and take the K-member with all the front end parts on it. You can find them pretty cheap. It will be the best and easiest way to do that type of upgrade. I live here in Tucson and there is the whole K member with front end for $150.00 that would solve all your problems finding something like that. Really if you want your boy to just cruise the slant6 will do just fine. Thats what i did for many years until i decided to do the swap. I swapped in a 360. but if you want to convert to 318 or bigger small block, find the K member and a 318 with 904 Tranny. The transmission in the slant 6 will not fit to a 318 . You could also install 1995 and earlier tranny into the 318 I would try to find A500 tranny. it will bolt to a 318 LA block. Hope this helps or i just rambled! Good luck