A 940 fan is a puller fan and the way to go for simplicity.
Without getting into all the nuts and bolts, this conversion is not going to net you any substantial gains in either hp or fuel economy.
Having done it about 2 yrs ago, my biggest gripe is the temp switches in the radiator go bad far too frequently IF you want to maintain an even engine temp, as these things swing in performance with age, and eventually go bad.
I think a major benefit is evenly running engine temps compared to a clutch fan, and also cool down when it's very hot.

I haven't taken an ammeter to the fan when it's running but it's a whole lot of current. The high speed is more than enough cooling in the hotest of desert heat, so the low speed is good for the general cooling of the engine.
I would bet that when the alternator is charging after a hot day of A/C idling and cool-down, the electric fan/alt draws a bunch more hp from the engine than does the viscous clutch fan.
Don't do this conversion without a good 80 or 100 amp alt.
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'89 245 sportwagon, destroyed by hit & run driver, RIP. '04 V70 2.5 T Sportwagon, 12k mi and '91 245 5-speed, 209k mi, replaced the '89