Upside, cheaper than an NV4500
Downside, no adaptors available and the OD isn't that great IMO. 0.85:1 isn't really enough of an overdrive to make a meaningful difference.
My old SM465, 4.56's with 37's...2690 rpm @ 65 mph
3053A, 4.56's and 37's...2287 rpm @ 65 mph
NV4500, 4.56's and 37's...1961 rpm @ 65 mph
I did look into the Spicer before committing to rebuilding the NV4500 though.
The cruise rpm difference was the deciding factor for me, seeing as both were going to be a fair amount of work.
Rene