Add as much HP as you want, the stock frame will handle it as it with street tires. Full out race truck will require some modifications, but not a street driven one.
What is the goal for removing the cross members? Just to remove a little rust behind them? For looks?
Have you tried getting a rivets out? It's a lot harder then you think.
Reaming out to the hole to the next size bolt with shoulder is the only way to bolt in a cross members, if there is any movement between bolt and hole, they will work loose. Hot rivets is the correct method. As far as welding them in, I believe you are weakening ( stressing ) the whole set up far more then you strengthening it.
There is a good reason the cabs are attached using spring mounted bolts and rubber pads, frame flex, its designed into them for a reason.
Good luck,
Joe
