It is quite simple to convert a '51 from Harvard to von Neuman architecture, all you need is a NEGATIVE or gate (-08 for example) use !PSEN and !RD as inputs and use the output at !OE on your memory. This, of course make it 64k total memory rather than 64k data and 64k code. I have probably made about half my '51 jobs this way. It simplifies many things e.g. you can use the same routine to output constant and variable text. DO remember to add something so the RAM or the ROM chip is selected only for the address space it covers (no overlap).

Have fun,

Erik