if i remember correctly you may need to just check the battery cable
feeding the starter motor , as it runs very close to exhaust manifold ,
try and tie it back away from manifold.

i actually had mine burn out the wiring loom in this area once due to
wires touching manifold and melting and then shorting out .

you may have old stickers that were taped to exhaust which are making
the smell, or it may be oil .

i allways liked the rimmer bros stainless system but i had 3 cracked
centre boxes exchanged on mine before i sold it for a 300tdi landy .

so long as i had receipt for proof ,  rimmer bros allways sent me new
centre box asap free of charge .
.
i cant fault theyre service thats for sure .

you may have some sealant residue on manifold that needs to burn off
before the smell goes away , or it could be plastic that was coating
the stainless plate they used to make the 4-1 collector on ends of
manifold