Wouldn't you weld mounts to the IFS cross member? Something like this: https://nolimit.net/prodinfo.asp?number=M%2D140
I bought Fatman's IFS and he sells a tubular mount that welds between the IFS crossmember to the boxed frame rails. He also sells another mount that welds to the cross member.
I am planning to put in an LS motor. It will require an adaptor plate to mount up to regular SBC mounts. I will set the motor down near the frame where I want it and tack in the mounts. There is a good article at classic trucks about fitting a motor into a F*rd frame, but it will apply to our cases as well, I believe. See
http://www.classictrucks.com/tech/0807clt_1952_ford_f1_hemi_engine_installation/photo_01.html and associated pics.
Hope this helps a bit.
