Usage is key, absolutely.
Do you really need a lot of floor massaging for that? (not saying you are wrong, just asking). I know that trans slips into a lot of older cars w/o issue. Just as a for instance the t56 in my 77 required all sorts of surgery, but a 4L80E in a 2nd gen appears to require none. I have seen on some older cars some fairly minor floor hammering depending on which 4L80E you get as the coolant lines were relocated at one point.
th400 vs 4L80E
My $0.02 is if it is a street car that gets driven more than 20 minutes go 4L80E. If it is just a drag car getting towed then th400.
I have not checked prices, but $2500 for a used TH400 seems steep to me unless you know the whole history of that trans.