727 Lock up converter choices?

The actual stall, in your combo is gonna depend a lot on your cylinder pressure and number of cylinders. So yes a 727 convertor will fit behind either a 318 or a slanty or anything with the right flexplate.
But the stalls will be all over the place, because the stall is a function of the amount of torque going into the TC versus the resistance of the chassis, behind it. and the torque of every engine is directly related to the cylinder pressure and how many pistons it has, firing on every revolution. This is why you can find catalogs listings several stalls with various engines but one part number.

A hi-stall 318 convertor will not be a hi stall behind a low-pressure slanty. But it will be higher than a lo-stall 318 convertor behind thye same slanty . Similarly, a lo-stall slanty TC will stall at a higher rpm when behind a 318, and even higher behind a 360.
Since you are starting with the least number of cylinders, and possibly with a very low cylinder pressure (IDK), you do not have a big off-the-shelf shopping window.
So bottom line is to consult a professional, or send yours out, or cheapest of all .........