i was in the same boat & didn't think twice about shelving my original drivetrain (383/727) for a 440/727 out of another car. it'll cost the same to rebuild the 440 vs the 383, so might as well get the extra cubes. plus, if something catastrophic happens to the rebuilt motor/trans, you haven't hurt your original motor. unless it's a showcar that's going to be judged on originality, i'd upgrade & get a replacement drivetrain.

my $0.0002