I did order the correct bolts for that cross member and have them in now. Spent this past weekend transferring the 75's passenger inner fender to the 77 also.Those look like the right bolts. I wouldn’t fret too much about it unless you’re doing a concourse resto. Probably find a flange head bolt at the hardware store that will work fine.
Of note to anyone else doing this, the 77 inner fender mounts to a small triangle type of bracket at the firewall.There are three bolts for that bracket. However, the 75 Inner fender has a different mounting system so the steps to use the 75 are that you have to first remove the 77's small bracket from the firewall, Then bolt the bolt the 75s longer bracket that runs up the inner fender. You will likely have to elongate the inner mounting holes so the bolts will match to the holes. I used a steb drill bit and opened them up and used washers with the original bolts.
After that, everything else mounts up like the original.
Other things I did this weekend was finish the passenger side subframe to radiator bushings, repainted that inner fender, replace the hoses to my heater core which were not there when I bought it and I replaced the bolts to the radiator support arms with OEM bolts I ordered. A lot more to do, but those were some big steps toward the finish line.