The key thing to check out is the torque delivery for a TD rather than comparing HP, so the Isuzu TD is likely to be much more sprightly than the old GM 2.8.
Example comparison - 1991 Opel Kadett TD (1.5 Isuzu engine) has roughly the same HP (mid 60s) as a 1991 Peugeot 205 NA Diesel. 0-60 in the Kadett takes under 12 seconds, top speed roughly 105. In the Pug 0-60 takes over 16 seconds and top speed is just about 90. Max torque in the Kadett is 105 lb/ft, the Pug only had 88.
I had a Kadett as my first TD car and it was a sprinter. Then I had a 205 which although it was more comfortable, better made and handled like (better than) a sports car it was definitely much slower - a slugger really. You didn't ever overtake in the Pug but in the Kadett it was simple to just prod and blap past anything without changing gear. Its addictive - my current car makes 230 lb/ft
The key to this conversion will be gearing. The gearing for Petrol will be lower than the Diesel will like. Remember with a Diesel the rev range is lower - typically only up to 4,500 or 5,000 rpm for older IDI/mechanical pump engines. Diesels have a limiter to prevent over-revving. BMC had a 1.5 Diesel saloon meant as a taxi in the 1960s and the engine would push against the limiter in top as it still had the petrol car's ratios.
Trucks (as in semis) have narrower and more gears because they redline at 2,000 or less in some cases so they need gears to give them a decent on-road speed range as well as the ability to pull large weight.
The lack of gearing can ruin a conversion - I drove a conversion of a Pug 1.8 TD into a Suzuki Vitara in the early 90s and it was superb up to 60. After that the engine was pushing up against the limiter and wouldn't go any faster - the gearing was setup for the standard 1.6 Petrol the Vitara came with from the factory.
If you are going to drive slower than that then this may be OK for you. You should aim for roughly 20-25mph/1000 rpm in top to give a decent cruising ability, lower depending on the weight of the vehicle and how much you use it full vs empty - flexibility when laden may be more important.
There are loads of other things to take into account such as fuel delivery, fuel heating if you are in a really cold area to prevent waxing. Payback will depend on cost. Doing it yourself in your own time might make it worthwhile.
What in the US had the Isuzu engine ? In Europe loads of GM vehicles had them from 1.5 NA to 1.9 TDs. You may find them in South America where GM makes some European style cars ?
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65MPH with a strong tail wind. had 4.38 axle gears. Weighed about 1850 on the scale at the log scaling station by my house.
