If it were mine I would take both heads to a competent machine shop and have the valves and guides checked. I have been building/racing since the 60's and have never stuck a valve in Bronze Maganize guides, which is what I run. The only shops who work on my machines depending on their back log are. "Katech" Parts only "Lingenfelter" (206) 727-2552 IN. My LS7 heads were done by "West Coast Racing Heads" CA (818)705 5454. Whoever did the heads may have not honed the guides enough. I have seen others who stuck a valve because of not enough clearance. However, that occurred when the engine was hot. The previous owner obviously rolled the engine over five if not more complete cycles to have five stuck valves. As you stated you rolled her over therefore depending on the valve relief in the pistons; the valves may have very likely come into contact with the pistons. Personally, I would rebuild both heads replacing the valves and guides for safety. Break or stick a valve @ 5k or better and you will likely be replacing the entire engine.
As far as timing, one could always take a felt pen and mark where the distributor was "Or" if you didn't; when you feel the valve issue has been addressed to your satisfaction. With the valve cover off and plugs out install a socket to the crankshaft bolt. Roll the engine over in the correct rotation until the intake valve on the #1 cylinder opens and closes. Look for TDC on the crankshaft dampener and if it is at or near TDC that will be TDC on #1 cylinder. Install the distributor and wire in the firing order, following the correct rotation. (if it previously ran and if you didn't change anything on the distributor cap it should be correct) It has always been close enough to fire, I always have my timing light and the open end/distributor wrench ready for the bolt holding the distributor.
