If I may add, sub-frame connectors should negate worrying about replacing the tunnel 'U' piece you cut out, especially with a beefy stock style cross member either fabricated like gdonovan did, or, connecting the cross member into the frame connector in a 'T' type fashion like so:
Taken from an episode of powerblockTV stuffing a 426 hemi into a 74' Duster. Crossmember fab comes in at 6 mins or so, which also gives detail on wall thickness of steel used:
http://www.powerblocktv.com/player/show_player.php?ep_show=MC&ep_num=MC2012-11
I've gonna say, if it's strong enough for a 426 new Hemi, it's strong enough for a 360 running a 42RH. :glasses7:
:burnout:
Also found another style of cross member fab on this episode:
http://www.powerblocktv.com/player/show_player.php?ep_show=HP&ep_num=HP2012-07