Do you mean a spacer like this?
http://blp.com/cart/index.php?main_page=product_info&cPath=72_126_304&products_id=1378Combined with this?
http://www.ebay.com/itm/Holley-4777-650-...y-/261997344277$260 would be pretty cheap if it works well. If someone tries it and finds out I might be putting a nice 390 up on Flea-Bay at the $260 mark and see what happens...
The secondaries on the spacer would need to be opened up to allow full bore.
...or I can make one with my buddy around the corner...
Now I wonder where I can find a 650 I can experiment with... Hershey's coming up! I'll dig around there.
edit: Wouldn't that kind of tapered spacer just increase the velocity without reducing the flow very much? I don't think it's as simple as tossing in a spacer like this. The bottom holes would have to be much smaller I'd think.