It's up to you, without seeing the frame its hard to make that call.
Chances are that the stuff under the cab and greasy engine are OK, it's just the tail end out in the weather that got rusty.

I'd be a little hesitant to replace the rivets with bolts though. Be best to re rivet everything back together. If you do use bolts use flanged frame bolts and the flanged self locking nuts. Most big truck and trailer parts places have or can get them, and they are not real cheap.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-