there was a lot of AT545's put behind the 454 engine so finding the adaptation parts will be easier than the MT643. the rpm is limited to 3800 with it but you may be able to tweak the shift points up a little (200-300 rpm) but, the downshifts might get a little harsh, especially the 2 to 1 downshift. you can set each shift valve individually (1-2,2-3,3-4) so you could set certain gears a little higher and the lower gears lower to give smooth downshifts. you can use the Allison vacuum modulator instead of the electric modulator since you have a gas engine. there are 2 torque convertor choices 275 and 290. the 290 is a low stall for diesels and the 275 is for gas engines. you could find any good used AT545 and change it to the rpm rating that you want with a 275 torque convertor, valve body springs (if the valve body is a reduced modulation you will need to turn it back to a standard modulation), governor and, vacuum modulator. the speedometer gear might have to be changed also. you probably have a cable type speedometer? if you do then the most commonly found AT545's have the 16 tooth electric speedometer gear in them. the rear bearing can be removed to change the gear to the 6 or 8 tooth for cable driven speedometers.
