If you weld to the frame, you will destroy most of the flexibility that is designed into the frame.
Bolt it.
And bolting a 'rigid' slider assembly to it wont affect it's flexibility?????? (this is me politely disagreeing)
I'm assuming you'll be welding to the outside of the frame, with the gas tank within the frame rails, yes? Then I wouldn't be to worried, but it is YOUR butt on the line. I plan on making bolt on ones eventually. Bolt on, so that when they get damaged I can take them off and bend them back straight.
I think I will build them, tac' them onto the frame so then wont move, then drill everything so the bolt holes are lined up 100%.
Also, since it's likely to get brought up, it is A-OK to weld to the frame on automotive stuff of the ranger/B2 vintage, and older. Disconnect your battery and make sure you have a good ground and your good to go. On newer vehicles I would unplug the comp as well.
The "dont weld to the frame" myth came from heat treated heavy truck (semi) frames.
PS: make sure your bolts are the right length, I bolted a shaped piece of plywood down in the back of my B2 and I used a bolt that was too long and it ended up rubbing thought my gas tank, leaving a 1" dia. hole!! oops!