The Stovebolt Site Forums The Parking Lot Big Bolts '54 630 shorten frame 4'

There are quite a few different types of those breakaway bolts.
One style - Avdelok® Lockbolts have a special nut and an extension that breaks off after torquing.
Then there's Tork-Bolts, which are like Ed was describing. They have a double head with the hex part that breaks off at the prescribed torque, leaving a rounded head that looks like a rivet. But the nuts are standard type, so could theoretically be removed if the head can be prevented from spinning.
Then there are shear bolts, like are used at the base of highway signs, that shear off when struck, allowing the signpost to release and not cause a bunch of damage to a vehicle hitting it. Those wouldn't be appropriate for bolting a crossmember to a frame though.

I would think that standard grade 8 bolts would work fine, if attention was paid to getting the hole diameter very close to the bolt diameter. The preferable method to do that would be to drill the holes undersized then clamp the crossmember in place and ream the holes in both the frame and crossmember to the correct diameter. Then tighten the pi$$ out of the bolts.