Kmot , I finally tried the OS FR%-300 carb on the asp 400. It is not a plug and play swap. Here are the #

Os carb barrel ID .2750 ASP .2775
OD of rt. angle ASP carb mount .3930 ID of the OS carb .3440 above the o-ring.

All the screw holes line up and are even the same tread and length. The os spacer is identical to the ASP spacer. There might be a slight interference between the OS throttle stop screw and the right angle housing be this can easily be solved. It boils down to needing to enlarge the OS carb case bore about .050. This would probably destroy the o-ring groove on the OS carb. Perhaps .025 off the right angle adaptor and .025 out of the carb. bore. May try this soon and let you know. The other alternative is to see if the carb mount off the FR%-300 would mount right up. I will research this and see what I can find.