A518 with No First Gear (so it seems)

So, since you didnt mention reverse issues,reverse must work. That means the L-R band is working.Since you didnt mention any other shifting issues, the governor is good.
That leaves just three possibilities; 1) linkage adjustment, and 2) valvebody issues, or 3)mismatched parts.

Since the 74 never came with a 518, there is no A-body linkage for it. So Ima gonna assume your 518 is an early model, that is automatically hydraulically shifted into O/D, and you have a O/D lock-out switch mounted somewhere. That means you could theoretically use the stock shift linkage.
So far so good?
Okay then, here we go. Jack the car up. Safety first, right? Rear wheels off the ground. And high enough to get yourself under it.Put it in N-for-neutral(a bit of humor there to take the edge off whats coming), and start it up. Starts ok? Good. Rear wheels not powered up? ok. Shut er down.
So, that proves N on the indicator is neutral in the tranny. Are you having fun yet?
Ok, so, leave the shift lever in N(for neutral), and slide back under there. Remove whatever locking device is used to attach the shift mechanism from the shift arm. What you are gonna do is verify that the low position on the valvebody can be reached.So you need to shift from N to D to 2 to L(1), thats 3 clicks;D,2,L. The direction is forward. So go, now.Badaboom, all three clicks there? Excellent. So that means the manual valve is working.
So now with the lever all the way forward, which we know is L(for Low), Push the shift linkage as far forward as it will go, and try to slip the pin back into the shiftarm. You may have to crawl back out and move the stick manually into low, then try again.If it drops right in, than Ima gonna say the problem is internal. But Ima betting it wont.
So, now we have a problem. With one of three solutions. Either 1) the linkage needs adjustment, or 2) the linkage anchor is too flexible, or 3) the wrong shift arm has been installed. This is the part you get to figure out.

I have to tell you though;this adjustment is to prove manual low selection.When you put the Mopar trans in D(for Drive)(sheesh give it up already), it always starts in low.Its built that way.If the tranny was assembled correctly, and at one time, had automatic-low(called Drive-Breakaway), I cannot imagine how it could quit,mechanically.
And that takes us back to an internal problem, maybe two internal problems.
Okay thats all I got.