You just put a cap on it and reference the mixer to the convertor via one of the ports. Because the 425 mixer is vac controlled on the power circuit the mixer needs to get it vac signal from another place rather than at the throttle plate.
I have not done it myself but have been told it is very very easy.
Tuning is exactly as if it was not boosted. Same process, EGA in tailpipe and read the meter. A guy who knows everything about IMPCO and turbo stuff is Franz Hoffmann in Texas.
He was very helpful to me when I set up my twin 425's on my 460.
His email is firstname.lastname@example.org
. You can tell him Tony Kuiper with the 460 Ford from Adelaide South Australia said he may be able to help. He is a good guy seriously. He is a lecturer in alternate fuels for Texas Uni.
There is also a book by SA Design called "Propane Fuel Conversions" It is book number SA12. It is in libraries so you can have a read. It has a lot of stuff from the 70's but goes into a lot of detail about turbocharging.
Impco themselves could probably tell you too, but they arent too interested.
There is also this independent IMPCO forum.
You will get a hit from Franz if you post there.
Prices?? My mixers cost 100 bucks each and the covertors were about 75 from memory. The mixers have new idle diapragms at 15 bucks each but thats it.
I did a lot of porting on them to get them to flow more but with boost you dont need to.