When looking at the specs, ignore the advertised duration (the longer one) as this is not indicative of the true characteristics of the cam. Instead, look at the duration at 0.050" valve lift, that is when all the slack/flex in the valvetrain is taken up and when flow actually starts to happen. So if advertised duration is around say 280deg, then duration at 0.050" might be about 210deg. Lobe separation can indicate how much overlap the cam has but not always. It also depends on the shape of the lobe. Nevertheless, it's a general indication of how much overlap there might be.
With a given duration, if you have say a 108 deg LSA, then it will have more overlap than 110 or definitely 112. The narrower you go in LSA, the more scavenging effect at higher rpm but at the cost of low-rpm torque due to exhaust gas actually bouncing back into the cylinder, and will give a rougher idle. Scavenging needs a good exhaust to work, so running stock manifolds and single 2-1/4 with a wild cam is a silly move.
If you go wider in LSA, it will tame the idle somewhat and be more torquey but will start to run out of puff sooner. Go too wide and you'll drop volumetric efficiency because the inlet is open for too long and mixture will be pushed back out of the cylinder before it closes again.
Playing with cam timing alters the engine speed at which the cam works best. Advancing brings the rpm band down, retarding brings it up higher. A barely tolerable cam in a 351 might be too hairy for a 302, general rule is that things happen 500rpm later in a 302, so torque and power need more revs to start happening. You could advance the cam a little to compensate for the smaller engine but I'd just go one step down in cam size and install as instructed.
One thing about Clevos is that they like more exhaust duration, due to the fact that while the exhaust flows quite well, it's quite a lot less than the inlet. It's less than the accepted formula of 70% (or thereabouts), so to compensate, they make cams over a certain size with what they call a "split" duration, where the exhaust is held open a bit longer than the inlet to allow more complete scavenging.
