Wow, I'm breaking a sweat trying to figure this one out.........I admit being a little partial to the 2.8. but it plays no part in my decision......I have owned my '84 B2 for almost 17 yrs & I have owned a lot of '84s & '85s consisting of both B2s & Rangers. The 2.8 is an awesome engine & the '84 you have is SWEEET!!!!!!
IMHO, If you will have to sell the '84 to get the '88, I would fix the '84! Although, I would gladly give you $300 for your '84, the way it sits, if you were closer!!
Anyway, you do not know exactly what you are getting in the long run with either the engine or the other B2, but with money being an issue, I would go the '84 route. I think you will come out better in the long run.....just my 2 cents worth. I would definitely throw a bunch of questions to the guy with the "rebuilt" 2.8 for $250 though...............Good luck either way!
B2