If I was buying new axles, I would not waste my money on buying 31's.
If I had a choice I would not even attempt it. just think about what happens when you break a axleshaft.
I have had a few freinds "give up drag racing completly" after breaking one.
You might want to check prices between the 31 and 33 spline stuff. When I ordered mine ( local guy near me is a Moser dealer) the 33 spline wasn't that much more and the 31 spline stuff was also back ordered. He ordered the 33 spline stuff since it was in stock .
1.35's all day in a 3450 lb fbody on et streets. Had a Moser 12-bolt with a spool, 4.10 ratio, and 31-spline axles. th400 so it was a pretty nasty launch although it never really dead-hooked.
the c lock stock replacement moser alloy axles are great pieces, and Ive installed many on some very fast cars. all I'm saying is if I had a choice, and was buying new axles made to length with 9" ends on the housing, 35s is the way to go.