option 3
i think is you best bet,
option 4
i would forget trying to bodge a set of 341's to look like 300's because real CB750 fans will spot them a mile away,on closer inspection they are very different inside as well as outside, if you gonna use the bike on the road you may as well fit the later pipes (341) , i have done this myself with one of my bikes its your choice really on how original you want the bike as it will all reflect on the sale price anyway this always seems to happen with projects, they either run out of money or rush to get it finished and cut corners as you can see on this site some owners here are very meticulous with their sandcasts and get them right down to the last detail, bearing in mind yours is not a sandcast and will not be the same value, you have done a pukka job so far, if it was my bike i would put a set of stamped 300 pipes on it but that's me, its your call, none of my bikes are show bikes , just there to be ridden so i don't worry to much about them being 100% original
option 5
is a none starter you would need to give me loads of money and body parts , i have a spare set of 300 pipes, i bought them a few years ago i knew they would only go up in price or they would dry up altogether, its a good thing they are still making copies, lets hope china can cash in like Vietnam