My current plan involves making the "back facing" exit point downwards for clearance so I can do a turbo project. They are off a 1996 Suburban with a 350.

First off, I'm wondering if these are grey or nodular. I honestly think I am dealing with grey iron here, just based on the way the metal behaves when I'm cutting it.

Second off, I need to add some steel plate to these. Is that going to be plausible to do without them getting crack prone?

Lastly, I have a MIG welder. I figured I'd get everything cut, make some nice tight gaps on it, throw it in the BBQ for a bit, MIG it up, whack the weld with a punch/hammer a few times then throw it back in the BBQ and let it burn the fire down. Any reason I should/shouldn't do it this way?

The vortec manifolds won't fit a thirdgen

I would say, forget it, you need nickel rod to weld gray iron.
Then it still might crack.

Thanks for the heads up don was afraid they were gray.

Fitment is why I wanted to cut and weld, also note I said turbo setup. I am going to flip, rotate and otherwise manipulate them. If I can weld them.

ive welded them before but not with a mig,
we would heat them with a rose bud tig weld them up with nirod an then hit them with the rosebud again and throw them in a bucket full of sand and then dump more sand to cover them to make them cool slowly

never had an issue with them

if ur doing a turbo setup build logs like i did , flanges frm summit for 35 bucks
and get 8 90* weld els and 8 45* weld els use a 2.25 or 2.5 inch heavy wall tubing for the log sections they will outlast the car when u are done

Most later manifolds are actually cast steel and weld quite easily without doing anything that special. That said, I have not tried welding vortec truck manifolds so u dont know for sure what ur dealing with