How'd Homer Simpson say it?   Dough!!! (kinda like that)
Sneezles61

I just did my first one on the LT. Bought it new from partzilla so nice and soft. I used my thumbs and like others said, ended up pushing it right on through on my first attempt and it was a bear getting it back out. I went with the same thumb push method the second time but made sure I had better control and it went in pretty good. I used some yamabond on it, the black silicone kind not sure of the number but I think it’s 3. It made it slippery so it goes in easy and adds a bit of extra sealing abilities. 

I just did mine on my 73 RT360, i put a little grease around it and she went in nice and snug. I need to replace the one on my 75 DT250 but to my surprise i didnt see any on ebay? The 74+ models are a different size, do they not make these new? im yet to really search for it as im neck deep in other projects.

Try HVC Cycles and other aftermarket places for the later sight glass.