Retractable 3-point harness??

I'm looking for some modern seat belts to put in my '70 Duster and came across these harnesses from Corbeau, they look pretty interesting...

[ame="http://www.amazon.com/Corbeau-3-Point-Retractable-Bolt-In-Harness/dp/B000FQ1QW0/ref=sr_1_40?s=automotive&ie=UTF8&qid=1339442748&sr=1-40&keywords=corbeau+seat+belts"]Amazon.com: Corbeau 2 Inch Black 3-Point Retractable Bolt-In Harness Belt: Automotive[/ame]

I know it's not recommended to run harnesses without a roll cage but these look like they would allow ample head movement in case of a rollover due to the 3-point design with the retractor at the top instead of the traditional 4-point type with the simple pull-straps that don't allow movement once tightened.

I was thinking of using the stock lap-belt mounting points for the bottom then mount the retractor at the stock shoulder-belt point; this way I don't have to make a new mounting point for "regular" 3-point belts to anchor the retractor down next to the back seat (don't really have access to welding/shop equipment at the moment). What are your feelings about this?