a lot of people through the years have actually found the conversion pushrods to be a little short, depending on factors (original deck height, head gasket thickness, cam base circle, etc)....
my guess is a combination of the magnum engine spec for deck height is 9.56", while the LA's is 9.60", IIRC, coupled with standard fel-pros head gaskets in the rebuilding kits are .054" compressed vs. ~.025-04" for OEM head gaskets originally installed.
some other options would be these:
http://www.summitracing.com/parts/EDL-9654/http://www.summitracing.com/parts/TFS-21407650/http://www.summitracing.com/parts/SUM-1457650/a pushrod length checker would be the best bet to figure out exactly what length you could go with.
for my roller cam motor, I was able to reuse the stock magnum pushrods, even with my reduced base circle (reground) cam. I had to use the roller lifters from a magnum cam, as my '86 lifters do not have provisions for pushrod oiling.
