I think a Chevrolet engine in a Ford product is sacrilege.
A Ford crate motor can provide more than enough HP, if you stick with a Cleveland or Windsor platform it doesn't involve new-territory problem-solving and fabrication, and will raise far fewer eyebrows than a Chevy engine.
That said, on as rare a beast as a GT5S (only valid if yours is genuine and not a clone) I would advise against ANY engine swap.
Larry
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.