Another question, when the body is on the frame is this area aft of the motor mounts easily visible from underneath so inspections for cracks can be done?
Yes
I went to a website called metalsdepot/com and for a a 6 foot section of 4x3 1/8 inch wall structural steel it is 55.00 a piece. So from aft of the motor mounts to a area for the bumper mounts is 6 feet enough?
I'm not a fan of Metal Supermarket - the price you pay for 6' is generally the price you'd pay for a 20' section from your local steel yard
Or should I go with 1/4 inch wall steel?
1/8" is fine, simply fishplate the joint (putting a piece of 1/8" or 3/16" flat plate over the top of the weld)
I thought about grafting the C4 front frame but heard it is a different metal then what the C3 frame is.
It's steel, the arms are aluminum, but you're not welding to those
I've never seen how a C4 engine cradle is attached to the frame however I never see professional builds try and graft the c4 frame to another frame. They always fab in new steel then make cradle mount and shock mounts.
Because it's easier to move it if you bolt it
Also would there be a issue with just welding in the cradle verses bolting it in? Or was bolting it in just done so the factory could install the engine and trans from underneath the car as an assembly?
probably
Here is basically what I had in mind, obviously I know this isn't a C3 frame but same principle. Any differences between frame and cradle could be fixed by welding in plate metal to shim it into position correct?
carefully consider the strength of what you're doing. Flat plates are no where near as strong as boxes
I though about eliminating the cradle but prefer to run a C4 style front spring on my car instead of coil-overs
Good choice on keeping the spring - ground clearance is something you'll need to check
Also I plan on using a LT1 4l60e or L-98 700r4 .... So could I eliminate the C3 engine mounts and use the C4 engine mounts and mod them so motor stays in the correct C3 position so I can use those bars right behind the cradle that attach to the cradle and frame?
If I understand what you're looking at, those are sheet metal plates - not strong enough. If you need to move the motor back, making offset plates out of 3/16" and move the holes back like we do with S-10 v-8 conversions would be a possibility
As a general overview of what you're doing - see how they made a very elaborate frame to hold everything square and at ride height? that is the most important thing you will do. You could do that swap with the frame in the car, but it looks, and works better if you have full access and can measure from all points without climbing under the car
I did that kind of swap on this car, I'm so glad I did the frame - made life so much easier
