radio for 73 Jav.....

No space to speak of under the seats in these - it would be tight under there.
 you can't see it to operate it so a remote would be a MUST.

If it had any power, heat could be a problem where it would get no air circulation.
I've had 300 watt amps under the seats in my Jeep pickup and I wondered about them on hot days. But then with ten speakers, two subs, two amps, an EQ and head unit in that little truck, where else could one put the amps? HA. No place else except back in the box.
Even with a remote, under the seat is really one of those less than ideal places. Heat, DIRT, lack of access, etc.
73 doesn't really have a glove box - in fact the thing they call a glove box is worthless, can't be accessed by the driver since it faces almost squarely to the passenger, and it's not big enough for even a standard DIN system. So that's not a player in a Javelin of this vintage. They are so small I can't even get my winter gloves in there. They are what, 4" wide?

(and it IS going back together this weekend, radio or no radio.)

Uh, question -  the inputs or ports are also rear-mounted. How do you access USB or aux input when they are rear mounted - and the dash is closed up? Really?
This is from their site - 
A rear mounted USB port allows you to play MP3/WMA music files...........
Two rear mounted auxiliary inputs for portable devices are also included.........
Rear?

Nice that it fits the dash, but at only 25 watts per channel with the noise this car makes I wonder if that's enough?
I'll be putting in only two speakers - it's all I can budget for now and all there's space for.
They'll be in the rear deck. there's no place else for speakers short of cutting up the door panels) 
I guess they have you since it's about the only shaft mounted system on the planet.

I hope to have two speakers - will be in rear deck as there's no door panel cutouts. That's also where he had the cable run to - the trunk area, from the dash area.

Otherwise it was likely originally an AM only car.  Currently there isn't a speaker in the car at all and no place for one but in the middle of the dash.

Now if it was the little Eagle - that would be different, but it's already setup for DIN so I don't even need anything special - although the next system I put in it will be a SHALLOW DIN with NO CD (who uses those in a car any more) A standard depth system smacks the defroster duct and you can't make it solid in the Eagle dash - there's simply no space and no way to brace it. The Javelin has room to swim compared to an Eagle dash radio area. Too bad the radio area of the Javelin is so NARROW. Tall, yeah, but really narrow left to right.