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I have a 71 bus with a 3 rib transmission and want to know if I can use a 3 rub bus transmission from a 72-74 bus without any issues.

You will need to use your 71 bellhousing and possibly nosecone mount. Easy swap.

I just did this. The later 002 bellhousing (with the mounting "ears") is not a problem, you don't need to change it.
The longer input shaft that 002 trannys used with the Type 4 engines may very well not fit
all the way into your gland nut. I've heard stories of some who managed it, and I tried, but the trans just
would not go in that last 1/2 inch. I had to get a new, shorter input shaft from weddleindustries.com and switch it.
You could also shorten the input shaft with cutoff wheel on a grinder I suppose, but I forsaw a big mess if I
tried to do that, trying to get some sort of bevel on the shaft end. Hacksaw blades I had just slid off the shaft,
so it appears to be hardened metal.

Input shaft shortening I forgot about. I use a thin cutoff wheel and use a bench grinder to taper. The later bellhousing doesnt have the pads & holes for the engine mounts.