Ray, ever think about interfacing a scientific calculator chip to Arduino? Back in the mid 70's I had a s100 buss 1k Altair computer and I got hold of a board that did that very thing. worked like a charm.
Yes., thought about it.. Back a few years ago when I was playing with the PICAXE but the UNO came out and had enough SRAM to do IEEE floating point math and being a RISC core, it was faster than the PIC. Some of the PICAXE dudes went down the coprocessor route but I threw in the towel and went full AVR.
I played w/ an S100 Cromemco while working in Research. Pretty awesome. However I went down the less expensive road of 6502 until the 8080.
PS, this was just a one-off to show it could be done. If I ever get around to it, I will probably implement a RPN style stack and the HP67 verbs.