What size bolts or screws goes into the crossflow muffler hangers?
 

Ive tried everything I have and nothing fits in it right. I don't think its regular threads but the course bolts used like on fenders and such maybe (even though ive tried about 6 different sizes and thread pitches also). Im about to just run a regular bolt and nut on it.

does not appear to have threads and material looks too thick for sheet metal type screws. suggest you either tap it for threads or like you said, nut and bolt it.

It actually has threads I can see and is thick enough to support threads and a couple bolts I tried started to go and stopped in a turn. I remember taking out the bolts 20 years ago or so when I got rid of the crossflow muffler. No biggie im gonna drill em out and pout some heft bolts in there to hold my fuel pump bracket is all im using it for.

Briann, I've got my 74 on a rotisserie now so it was easy to check. I found that the crossflow muffler hanger, tail pipe hangers, drive shaft bumper, front gas tank strap & many or the fluid/vapor line attaching holes are all the same thread. I used a one half inch hex head course thread 1" long pointed tip. What I would call a heavy type sheet metal screw & was able to turn it in with my fingers in all or the afore mentioned screw holes.

Thanks! I havn't drilled it yet so will check it out when the temperature goes below 104 out there. Its a scorcher today and my swimming pool and 70 degree house gets the nod instead of car work.