So the part that you weld to your frame is just an angled piece of steel with a hole for the bolt:
10 Pack 2 Hole L Shaped 90 Degree Angle Connector Bracket Fit for 1/2" Bolt in 1-5/8" Strut Channel, Thickness 4.8mm, Steel Surface Electrogalvanized
https://a.co/d/02BmyCyo
You can buy unistrut cams at your local hardware store or Amazon:
1/2" x 15/16" Strut Channel Nut and Bolt Set (10-Pack)
https://a.co/d/0jb6x3pQ
1. Position the tank in place with a floor jack. Get it as high as you can but you do not want it touching the underside of the bed, there should be at least some clearance there.
2. Hold up angle brackets and martk position on frame.
3. Remove tank from vehicle.
3. Frame will be fully boxed in this position so your best bet is to weld the brackets to the frame.
4. Reposition tank in and attach with unistrut bolts.
-JB