Valves that are too tight won't seat well, which means they don't cool well, which will lead to a burned valve....
+1 backside method. Remove feeler gages from holder. Rubber-band a couple of popsicle sticks 1/2 way up around the feeler gages - that way they won't buckle when you're trying to weasle them between the cam base circle and the rocker.
It's a PITA. Keep at it, you'll get there. No shame doing it again next week - I did my 1st and 2nd valve adjustment in rapid succession
I re-used the nuts and washers for the 2nd adjustment. Most of the lower ones on the exhaust side valve covers ended up leaking & were later replaced. The rest are dry 1000 miles later.