I got mine from speedway and the welds are gorgeous, best 75 bucks I have spent.
I got mine welded in. I had the original mounts to measure from, so 22" off the floor to the front hole (dont use this dimension, thats just how I did it) and 10 1/4" rearward move. it sat a little over the frame in position so I built some close out pieces and welded those in too.
IMG_7415 by
Joe Doh, on Flickr
IMG_7418 by
Joe Doh, on Flickr
you want a 4 degree down angle, your intake is set this way and so is the trans pan. you may not be able to remove the pan with it in place. you will also want to offset to the passenger side for steering clearance! very important