Yes you can run a BB pump in the SBC and I've done it for years. The shaft is Moroso 22090
Moroso 22090 - Moroso Oil Pump Shafts Details
# Can also be used in SB-Chevy or 90° Chevy V6 engines to compensate for reduced distance between main bearing centerline and distributor shaft tang when block deck has been cut and main bearings line-bored.
# 400 cubic inch small blocks must be machined to accept these Oil Pump Shafts The oil pump shaft opening must be enlarged to a 9/16" dia.
An M-77 pump is not high volume.
Also, roundy round guys have been doing this forever, it is not done to mask or cover anything up.
As for any eatng up of a few HP, it wouldn't be noticeable in a 500+ HP app. I'd look at any improvement in oiling as a plus. If the SBC pump was so good, why does the BB pump have more spur gears? Why didn't GM use the SBC pump in the BB?