I,ve fitted a standard Sierra sized battery onto the Saturn bulkhead/ scuttle......as you have found....the standard size battery doesn,t sit into the moulded shape on the bulkhead,
BUT
If you cut part of the forward INNER face of the moulding the battery does actually "Slot" into the recess, along with a securing strap this helps to totally hold the battery in position.
Once the battery is in position you can,t see the bit that was cut away on the inner face of the moulding
I,ll try & find a photo for you .
Forgot to mention.......You will probably find that you,ll end up endlessly tidying up the wiring thru the course of the build, I know I was always adjusting this n that until the build was totally finished so left the final "tidy up" of the wiring till the very end ( In the engine bay I mean )
cheers
andy