What problems will i run into changing a 1964 slant six Valiant over to a small block 318. Thanks

This is one of the easiest swaps you can do. The K members are the same, just the engine mount brackets are different. You can get new ones through Schumacher for about $175 including shipping. If you decide to use a 904, all the linkage swaps right over. A 727 fits, but it is tight. The pinch weld in the trans tunnel needs to be flattened a little. To run dual exhaust tucked up tight, you will need to modify the trans crossmember since it only has a "cutout" on one side. Its not difficult if you have a welder and a grinder.
There are headers available, but unless you are using a motor with over 300 HP, the $700 cost may not be cost effective. 5.2 and 5.9 Magnum manifolds are the best flowing units besides the 68-71 340 parts. Look for the 1993-95 Full size truck manifolds.

mtr mts from a 64-66 dart/valiant/barracuda 273 V8, the insulators are the same as /6 but usually come w the mts when they're pulled. '66 (& some '65's) A body throttle bracket/cable assy which bolts in to the factory dimples w 3 new drilled holes but these are somewhat rare & a 70's common pedal assy will bolt in w drilling as you need the cable throttle to match the carb unless you use a 64 rod type throttle carb. Need 273 V8 ex manifolds esp the dr side (later pass SB ones are OK but will have a larger outlet which is better ex the dr side will still ha ve the 1&7/8 iirc so your pipes will b mismatched. 340 HP manifolds will fit if you shorten the steering coupler to about half it's orig length or if man steering use a PS inner shaft + the firm feel extension adapter (the inner shafts swap over directly, the snap ring under the st wheel is the only thing holding it. there is a great amt of tolerance stackup on early A's so mockup everything 1st as you might get lucky (once a 727 bolted in & barely touched the pinchweld) Use an iron early SB water pump if you want dr inlet & adj the pulley depth w spacers/washers. As Franken said must trim the trans x member otherwise the pipe will b very low/look ugly

IIrc a v-8 steering centerlink is needed..maybe sb torsion bars and a bit more cooling capacity

Why would you want to swap a /6 when it's the ultimate muscle motor? You just can't kill them!

OR turn a sub 18 second 1/4 mile time!

manifold clearance is tricky in the early a's for anything other than a stock motor.

the 273 manifolds to fit those will likely choke even the 318. not sure if a later a body v-8 drives manifold will work in the early a's but from what i remember the early a v8 drivers manifold has a goofy bend to clear the steering column. also very small ports.