I had spoken to Richard Calvers brother, Phill "Rastas F Ferret" Coltman when the came to the UK last year about this conversion. It seems it is a good conversion in Australia as it is a very common gearbox in Aussie GM cars, and rebuild and shift kits are cheap (the GM700 is designed for small blocks and needs a lot of upgrades to handle big blocks).

A GM 700 in situ in an Interceptor

Picking up a GM 700 from a scrap yard and rebuilding it is a cost effective option in the US and Aussie, but as they are not readily available in the UK it means getting one imported, and it then becomes the same price as a 518.

See the article below the Phill sent me.

Article 1

Article 2

Article 3