The DT 466 tachs out @ 2700 RPM's I wouldn't try to get that bus to go 70. You have a multitude of factors. Tire size, Differential Gear ratio, Transmission.
the other Automatic you are talking about is still a 4spd and may have a ratio that is slightly higher but is basically a heavier duty 4spd than the AT 545.
SALES Pitch. I have a 96 International THomas 3600 Vista with a DT-466
and AT545. This is a semi forward design. Basically it has a snub-nosed from end that looks like a standard school bus but shorter out front. It handles and steers nicely. It is white with some cool blue and orange stripes. It has 169,000 miles on it and is in great condition. Air Brakes, which are less expensive to maintain and repair. Everything is simple and easy to get to. I am soon to put some pics up. Basically I have done all the work to it and am still working on it. It has been fully sprayed with foam insulation and floored with another layer of ply wood. I am building extra tool boxes and an aluminum rack on top and nearing finishing the paneling. I have been set on making this a sweet bus but recent circumstances have led me to consider selling it. This is the first posting I have put up here. I have a whole list of info and research on anything you want to know about skoolies and how to find it. Equipment, prices etc. If I go through with selling, I will be posting it here first but likely looking to ebay with a minimum bid around $5500 as it is right now.