Just an update - the battery checked out through two load tests fine with well over 900CCA @ ~13V. Timing was a touch too far advanced @ 15' BTDC so I dialed it back to 12' BTDC, however this didn't help with the starting issue.
I finally had a chance to get the car jacked up and have a look underneath this morning. The starter connections were a touch loose, however the main issue was the ignition wire was close to the block and the insulation had worn/burnt off causing a partial short. Tightening the connection rotated the connection away from the block, (which is how I noticed) and resolved the problem.
Thanks again for the suggestions guys
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1970 Pontiac GTO
Orbit Orange Judge Re-creation / YZ RAIII 400 / M21 4-Speed
1985 Chevrolet Monte Carlo SS
5.7 LITRE / TES Headers / SShaker Hood / WCT5 5-Speed