I haven't done a 73. But I've done several 68-72. Assuming the front sub is similar.
You can do this with the stock small block engine mounts, The big block 67-69 Camaro..68-70 Nova frame and engine mounts work better, They position the engine lower and to the right.
But the small block mounts will work. Interference with a power brake booster is an issue. Not bad. Sometimes a dimple in the valve cover will cure it.
You will need to remove the heater core and flip it (as the factory did) so the outlets come out closer to the fender, Drill two holes in the heater box and re-install. You can by pass this mod. But the BBC head is very close to the stock heater outlet and will break the pipe if you try to move the engine around for install or maintenance.
Stock BBC's used a radiator that mounted to the sides of the radiator support as opposed to the small block top and bottom mount. A recored small block or aftermkt radiator will work.
Trans crosmember for 400 turbo was different from the std trans/350 autos. I see repros on Ebay periodically. The stock crossmember can be modified. Driveshaft will have to be shortened. Approx 3".
Stock rear end will not last with big tires and "spirited" driving.