This how to will show you one of the ways to prepare your front frame clip to add a custom frame.

After totally disassembling the truck and lifting the body and front sheetmetal off you will have your frame. The first thing you want to do is place the frame on evenly tall spacers and make sure it is level. This will put the front clip at the appropriate angle it needs to be.Then you will want to get your measurements where everything is (bodymounts, rear axle, ect) You can do this by removing the lower a arms and hanging a carpenters plumb from the holes.

Mark on the floor where the plumb sits and do the other side. Use the lines to snap a straightline across the floor.

Use this line as the main reference point for all of your measurements. I again hang the carpenters plumb from various points to the tape measure on the floor.
After all measurements are wrote down its time to secure the front clip. Weld pieces of scrap tubing from the front clip to the floor (you can lay a sheet of metal on the floor to weld the tubes to if you feel more comfortable). Be sure to weld them strong enough that whey wont flex.

After you have the front clip cradled, just cut off the frame haha.

I use the opportunity of a lightweight front clip to flip it over and trim out the spring pockets. After you are done hang your carpenters plumbs again and draw your reference line to get to a point where you can begin to build your frame. All you need to do is remember how high you spaced the frame during mock up and build the frame ontop of that measurement.

Good luck