1966 valiant v8 swap?

I am new to site also to mopar, i spent years racing a 65 comet, but i am now the proud owner of a 66 valiant, love the car, this one is solid besides floors, not sure how that is, lol, but i am putting in new floors doing all new bushings in suspension..... Point here is i have a slant six now, once major repairs are done i know myself and need a v8, found headers from hooker( pricey) i just need to know 318/340 what transmission can be used? Rule of thumb etc..... Last but not least very proud to be a mopar owner......thx