What do I actually HAVE?

[mmcclellan]

greetings fellow 'stang nerds....
i just picked up a hodgepodge car that i'd like some thoughts on. to begin with, this is no purebred, nor do i care too much if it is. that said, some of your replies may be personal preference which is okay and welcomed.
i just bought an 84 convertible body, outfitted with all kinds of not original jazz. the guy i bought it from said he totally rebuilt an 85 or 86 gt 5.0 and dropped it in.(dunno how you don't know what year you bought and refurbed, but oh well.) engine looks great, and it runs pretty well. it has a true positrack rear end with dual flowmasters. also has a 4 brl 600 holley carb. he said it has an old 'metric' transmission. i don't really know what that is, but he said he'd reco an upgrade as it has a lag between 2nd & 3rd (automatic).
my questions are these: any good tranny options? it has front ground effects which i don't think 84's had. what year is it likely from? (doesn't fit really true) how do i tell what year the engine is (or how much does it matter)? it has the cobra emblem in various places, which i also don't think was an option. what engine specs do i likely have?
any help would be great, as i'm trying to learn what i can & should do to it to find a balance between preference and historical accuracy.
thanks!