1964 Valiant Slant 6 to 318 Conversion ?

hello everyone,
so here's my dilemma i got this car a few months back and me and my buddy have been working on fro awhile. did a few things like rebuild=t the front end and converted to disc brakes. now we were read for the test drive and when i put it in D it didnt want to move tried reverse nothing. it didnt want to go into any gear. the fluid was check and was fine. to add to this the car was sitting for who knows how long it was supposedly rebuilt in early 2000s. my issue is do i have the transmission rebuilt or do i go full swap to 318 and if i do i would like guidance on that. ive spent a few hours looking online to swap and have many questions like the whole shifter situation. it has the push button ordeal. i know m cheapest option is to just have that 904 rebuilt but it wont be cheap. id like a break down of what parts i would need to do this swap. any info would be much appreciated.