I've got a 75 280 with 133K on the original motor (getting a little weak) and a 77 L28 with 80K on it, but hasn't been run in probably 10 years. Question is do I swap the old motor for the newer and then rebuild the older or other way around? I don't plan on doing any major engine mods, probably just get valve job on the 77 (B4 swapping) and some minor ignition changes. Clutch definitely has be redone, but there is a decent clutch on the 77 motor. Give me some suggestions. Thanx.

I'd go with what's working. If you want to refresh your existing running engine, you'll likely have more confidence in the results. Typically, L engines run well past your current mileage before they suffer any significant decrease in power. I'd re-do the clutch and keep on drivin'.