318-360 lock up torque converter swap

I have a lock up torque converter from my 81 727 that was behind a 318. I was planning on using a torque converter from a 360 that I have..
I thought the torque converter from the 360 was a lock up as well but it is not.
I'm using the transmission from the 318 for the 360. I know the 318 TC's won't work unless you add the weight from the 360 TC. Or get a B&M flex plate. Since this is a budget build I was planning on welding the weight into it, but two questions come up, where do you weld it and does the weight of the weld play into the final result. Also heard that you shouldn't weld the weight onto a lock up TC as the heat will destroy it?
Any thoughts? Greatly appreciated.

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