If I change out the stock driveshaft flange on an 85 axle will i be messing with pinion depth. If so, would it be a better idea to drill out the flange holes to fit the 10mm bolts, or is there another way to fix the issue?

 

I would redrill the flanges 45 degrees off. That way you can have the old bolt holes if you ever want to reuse the old bolts, and you will have the new holes as well.

I think its Marlin crawler who sells the triple drilled flanges, so its definetly strong enough.

 

If careful, you can pull and replace the pinion flange:
http://www.4crawler.com/4x4/CheapTri...eal/index.html

You can also drill it on the vehicle if you need to. As noted above, just place the new flange on, holes 45 degrees off the existing holes, mark and drill. I usually do one hole at a time. Mark, drill then insert a bolt in that hole, then mark and drill the opposite hole and repeat one hole at a time until all 4 are drilled.