Honda-SOHC

Hi new to the site , First post , been using the american site ,didnt know about this one.got a 77 550 with a smashed 3rd gear. just fit a new trans and drum & forks i got of e bay £80 , seems in good condition , just about to re join cases.
theres quite a long thread i posted on sohc.net.
 a bit worried about fiting primary shaft when i have re joined cases , not much room for starter clutch ect , is it best to loosly fit primary chain on sprocket first ?  when removing the primary shaft i couldnt get the chain off the sprocket untill i split the cases a little.
cheers Paul