The '88 model has far-superior factory suspension than the 84-87 models. It is much better handling and more adjustable.
Unfortunately, this is common knowledge, so '88 prices seem to be somewhat higher.
If rules allow, consider using a 4 cylinder Fiero and swapping-in a GM marine block 3 liter engine. It is externally identical to the 2.5, but much stronger and more powerful.
I have a marine block 3 liter 4 cyl in my boat. (It's an OMC, not a Mercruiser)
It's rated at 140 hp (50 more than the 4 cyl Fiero) with just a 1bbl carb!
It can also run at WOT, 4500 rpm (hp peak) all day with no problems.
As I recall, there are numerous hop-up components available for the 3 liter marine block, too, to give you 200+ hp reliably.
I have built a 2.8 V6...... and I think it will be less reliable for 12 hours than the marine block 3 liter.
The 2.8 design is somewhat limited by short intake runners and bearing oiling problems at sustained high revs.
I modded mine (Edelbrock cam, 4bbl intake, shorty headers, .080 overbore, Fiero heads) and it has about 150-160 hp .... I burned up the #1 piston after some heavy towing. I fixed it, but have been more gentle since. It's been reliable, but I have not thumped it.
Personally, I think that GM did not make a marine variant of the 2.8 because it is not very durable for severe use.