I worked in 'smithy' (a blacksmith shop) before becoming a machinist. I'm inclined to get OEM rivets, as they are still available, and follow the
FSM, (pdf) pg. 223
However, in my opinion, some of the frame
might be repaired with bolts. The front bumper is attached with bolts, and it is subject to frame twist, and the shackle hanger is right under it? The service-life of bolts should be seriously questioned. The severity of service should be questioned.
Using a bolt to fit in the same diameter of hole proper for a rivet seems like a compromise; enlarging the rivet hole, and modifying the frame, could be a better strategy. Grade 5 (SAE) with Grade B-nuts, or Class 8.8 hardware is commonly available. The use of a fine-thread is preferred over a course-thread. Fine threaded hardware has a greater minor-diameter (thread-root on the screw), so you achieve a greater cross-sectional area of fastener. Flat-washers, on both sides, the head, and nut are a must. A split-type lock-washer is desirable, in my opinion.