1997 GMC Sierra 5.7 Vortec

Just bought a 97 Sierra. Started and ran great
when I went to look at it. Apparently it had an
engine swap done just before I bought it. Been
about a week and 100 miles since then. About 10
starts no issues.

Now over the past 2 days it's had difficulty starting.
It will crank and crank and not fire. Then I'll leave it
for a second and try again and it will fire right up. It
began when I shut the truck off in the drive thru.
When I went to start it I don't think I let the starter
engage long enough and the engine fired but ran
very rough, then stalled. Tried to start it again but it
just cranked. A few tries later and it did start but
ran very rough. Had to mash the pedal til eventually
the rpms evened out and it ran fine. (Thinking i
might have flooded it?) Started normal for the rest
of the day then this morning I went to start it and it
cranked for about 10 seconds. Stopped then tried
again and it fired right up and ran fine, lots of
power.

The only codes being thrown are for 02 sensors.
Going to check all fuses and relays but wondering
what else it could be and if anyone else has had a
similar issue.