When you go to order side motor mounts, be aware that there are two different frames under tri-5's...a one piece and a seamed frame. You need to determine which you have and order your mounts accordingly.
I used McGaughey bolt in mounts, but I welded them in.
I used an Earl Williams classic chevy rear crossmember with a built in driveshaft loop...cool piece
You can keep your front mounts if you install bellhousing mounts from someone like Classic Chevy International or Danchuck, but then the frame horns will interfere if you go with a shorty style header....It is easier to convert to side mounts and a rear crossmember and whack the frame stubs out of there....