what engine to use

i have a 73 road runner iv got a 400 big block for it and i was wondering about stroker packegs for it that i could acheve around 500 hp and is prity durible the trany is a 904 that will go soon but my other option is a 360 small block but i kind of leaning to the big boy  

there are a few stroker options available for your 400. you pick your ci. 440 source is one place to check out. the 904 won't work behind the bb.  

Check out Muscle Motors out of Lansing, Michigan. They have stroker 400s in crate motor form and I believe they have kits if you want to do it yourself. These guys are highly informed on Mopars and make some really killer engines. Good guys to deal with.  Bill  

We have a a pair of 67 Satellites one with a 440 and the other with a 360. They are street strip set ups. Currently the 360 runs a tenth quicker with a best et of 12.56 at 107.5 while the 440 has gone 12.66 at 106.
My 360 gets 18 mpg while my friends 440 gets 12 mpg.
For me it was a no brainer, since they are street cars.
The 360 uses LA heads stock valves with a port & Polish as does the 440.
As you can see either engine works, its all in the combo.  
The only added improvements to the 360 is a solid cam and another 500 RPM stall in the converter.
However the 360 has a dual plane intake while the 440 has a single plan.
If you are going to stroke either engine you will get similar gains for both.
 

The 400 will make 512in and a lot more usable hp and torque if done right i.e. heads cam and carb.all on pump gas 90 -92 octaine around 600 hp with no problems.
The 360 can make plenty of power around 600hp but at what cost aftermarket blocks and soforth.If I wanted 450 hp i would go with the 360/408 if i wanted 500 to 600
i would use 400/512 it would be cheaper to get there. remember no replacement for displacement