Trailing arm length, C-Notch Placement

[52 Rat]

Guys, I just picked up a new set of NASCAR trailing arms this weekend. My concern is the U bolt size and the axle seat hole location. Keep in mind I am installing the arms in my 52 AD project using a '70 trailing crossmember. The U bolts I have are 4.25" not 3" like the stock axle seat. The guy I purchased the trailing arms from supplied the new U bolts and nuts. I need new trailing arm axle seats and from what I am seeing on line, they are all for 3" axle tubes not 4.25". What am I missing here? Any advise is greatly appreciated.