Ran across a 1970 I 6 ???" Maverick with ps today. There was a single groove water pump pulley and a 3 groove crank pulley. The ps bracket and pump was still attached. The ps bracket mounted to a boss on the head with two bolts and also to the block or timing cover with 2 bolts. The distance from the boss on the head to the outside of the Thompson pump was about 10". It sat directly above the oil filter. The water pump belt used the inside groove of the 3 groove and the ps used the groove closest to the radiator. The ps bracket looked like it may have been made to mount a compressor on but I am not sure. That was probably what the center groove was used for. I may be going back Sat. Pulleys are $14, bracket probably $10 and the ps pump $30. Didn't see any later model 6 cyl.
The Cardone/Napa/Saginaw is a bolt in for the Thompson pump but pricey