The 4.3l is 70mm stock, and has a restrictor/baffle on the plate so that it's not so touchy when partially opened.
IIRC the 5.3l has the same bolt pattern as the 4.3l, and I know the 5.3L has the same bolt pattern as a northstar (75mm) TB that ryan hess was selling on 60degreev6.com.
65mm is getting close if not the limit on the 3400 plenum.
The 4.3l /etc TB's have 3 bolts in a triangle pattern.
You might be able to adapt a 65mm ford 4.6l/5.0l TB to the plenum, and just use 2 of it's holes. I know that works for 3500's. But then you gotta fab up something for the IAC. (I hear there is aftermarket GM IAC adapters for ford motors for people using aftermarket computers).
Another idea is a Jeep 4.0L 60mm TB, I think the IAC is compatible with the ECM.
That's all I know of. I'm going to half the throttle shaft in my 3400 TB and thin the back shaft and cut the screws shorter for a few more CFM and call it good. I'm not sure that a bigger TB will net me more power even though my plenum takes 65mm stock.
EDIT: Another option is to take the 3400 TB to a local machine shop and have them bore it to the limit (60-61mm) and cut a new plate for it. Then you can halve the shaft and it should be pretty good then and still bolt up and work like stock but be pimp inside!

I've heard local shops charge ~80 to do something like this