Sorry to be negative as I like nice parts being made for volvos as much as the next guy, but I think you would be hard pressed to justify the r&d with the quantities you'd sell - think 2 or 3 per year in Aus. Given the 2kaud price tag for a manifold would be more than the cost of most of our cars, your only hope would be our shitty dollar and people in the US and Europe buying them. Even then our marquee isn't exactly renouned for spending any serious money on the platform, and when they do they are generally less chequebook racer (ie sr, rb, jz 4g etc) and more diy.
Beyond that, developing for a 8v head is probably trying to solve a problem that isn't there - the stock intake manifold isn't really a restriction until you make serious power, and then the 16v head can be swapped on for probably half the cost of a Plazmaman inlet manifold and have about a factor of 10 in performance gains compared. I'd say develop it for the 16v head but then there are even less potential customers as it is far less common.
The existing options are KL racing at about $700aud;
And a few limited runs / cast ones from overseas;
All the best, sorry to be discouraging but it is the truth in my eyes. My advice would be to develop diy kits like universal 4, 5 and 6 cylinder plenums that bolt down to a provided blank flange that customers can drill to suit their runner spacing requirements - like what Integrated Engineering, LPS and Jenvey do.
