standard pedal threading

[11.4]

If you placed your new pedal hole 3/4" from the original one, you'll have barely an 1/8" between the new hole and the old one. That's dangerous. You have a built in weak point with a serious stress riser ready made for you.

TiD is correct that you can't just change crank arm length in isolation, but I've fit a number of riders including some paraolympic riders with very significant discrepancies and I find it's a combination of crank arm length, cleat position, and cleat shims, plus a recognition that you may simply need a different crank arm length on both sides compared to what you may have had before. If you have a 3/4" leg length, you will make up a good quarter inch or more just with different ankle rotation. For the rest, size your cranks to your right leg and then get a longer left crank (the cheaper crank arm, plus it's easier to find longer than shorter). Don't let anything on the bike (like the saddle) become to asymmetric -- you are better off letting your legs and pedaling style compensate for some of the discrepancy than trying to do it all and just screwing up your back or hips or something else. Believe me, it rarely works out.

If you really want to try crank arm lengths, get a pair of adjustable-length Powercranks. They are available in a version that gives you something like 160 to 185 mm in 2.5 mm increments. Now Powercranks are intended to have you ride with the left and right leg independent from each other, but you can link them to make them work like regular cranks. Then at least you can try accurate lengths in a decent crank and see what works. We've used them a bunch and no rider has ever been happy in the end with more than a small partial crank arm length difference. And that includes some riders who happily ride triathlons and RAAM.

I'd find a really good fitter, preferably someone who has worked with paraolympic athletes, because most fitters are used to small discrepancies but have very little experience with something as pronounced as yours and experience is really important here.

[torquer]

The Look ZED is an interesting approach to adjustable length cranks:
"TRILOBE Crank System
There are no threads for the pedals. Instead there is a triangular insert for the pedal called the TRILOBE (LOOK patented).

The main advantages of this system:
•The crankarms are stronger without any CroMo or Aluminium inserts that weaken the area.
•The rider gains the ability to change crankarm lengths from 170 mm, 172,5 mm to 175 mm using the same crankarm

The KEO ZED pedals are included with a special axle system that works specifically with the ZED crankset."

https://www.cyclesuperstore.ie/shop/...asp?idpage=120

5 mm difference probably won't be enough for the OP, and being limited to KEO pedals may limit their appeal. Anyone know of another crank using a similar approach?