get the 2800 youll be really happy with it,
and you still need to pull the whole tranny out to swap convertors, but its not really necessary to change to better internals either, your stockers will be just fine, but to build a GOOD a604 you need to uprade to better internals like kevlar clutch packs,etc... that will help each gear grab better/faster which is good

and no I dont have one YET, but Im coming into some money and might just have one hell of a car by years end