I fitted a 1.7 5 speed M47R transaxle to my 1.4 1980 300 with the M45R about 20 years ago it is a direct swap , even the linkage fits , it cost me £5 ! The gear ratios of the gears are the same with 5th being an overdrive .
The final drive in the 343 was 3.9:1 , the one in the 1.7 is 3.64:1 so the gearing generaly was higher giving a relaxed drive at lower rpm with less torque of course , you dont have to use 5th gear but where you can it is worthwhile .
The final drive in your 1988 1.4 should be the same as the 1.7 ie 3.64:1 there will be a white label on the transaxle stating the final drive ratio .
Some data below .....
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 My comments are only based on my opinions and vast experience .
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