ok well lets start with dry fitting.
enlarge fitting holes on all chrome parts....even if stripping,...
File, sand, get rid of all sharp, parting lines ....(tops of front and rear 1/4's and , usually inside window openings,...) use yer feely thingy's to find sharp edges...
I use tape to hold things together whilst Dry fitting...(really,.... whilst?)
Sub assemblies can be built,.. test fitted before, say putting the headers on,.. to see if they will actually go , where they are supposed to,..Ya know ?
Have Fun, and MAcGyver on !