My 273 turned out to be a 318...what did I buy?

Sweet little car! I like it, a lot!
The engine swap appears to be pretty well thought out; with a collection of solid, well considered (but not fancy or over the top- a good thing) modifications- but as said, the execution could use a bit of cleaning up.
From what I can see of the build, only two things concern me once the wiring and lines are sorted out.
If it has the original '65 transmission, there will be a discrepancy between the convertor nose and the 87's crank flange. '65s were designed with a smaller diameter convertor nose, which was enlarged in '68 or so; meaning the pocket in the '87's crank flange is too large to properly support the convertor. There is a reducer bushing used to correct the situation that gets pressed into the crank flange. But if this was not done at the time of the swap, it could result in some weird vibrations or worse- a cracked flex plate. Now that it's together it is impossible to tell unless the PO can clue you in. But it is something to be aware of, and if something begins to develop, you'll know to stop where you are and correct it.
Second, the master cylinder. The originals are a single-reservoir design which was superseded by a dual-reservoir design in '67. The singles functioned well, but any brake leak or other issue does result in loss of the entire braking system- which is why the dual reservoir systems were developed. They split the system into front and rear brakes, so any issues that develop only affect that part of the system, and not a total loss of braking. A future upgrade to a later dual pot master along with it's matching distribution block should be on your "to do" list. Take it from somebody that has experienced a brake line failure on a single pot master- it's not the kind of excitement you're looking for when you go for a spirited drive.
Other than that, it looks like it's going to be a really fun car!