The Miata 1.6 motor is the same basic motor as ours. So yes the rods will fit. There are chinese made H-Beams on ebay for $229 that I have had my eye on, those should work nicely if the quality is there.
Welding the diff helps prevent premature diff pin failure, but if you do a lot of burnouts/spinning one wheel excessively the problem still remains. There's no bearings on the inside of the spider gears, so they overheat, gall & smear the shaft they ride on when abused. This will ruin the diff in short order. Go MFactory if you can.
450cc are "too" big to run on the stock fuel system. It'll run pig rich. You can combat this with a RRFPR, but I've seen mixed results with that. You can run a Tbird compressor on the stock injectors to ~18psi - but make sure you have a wideband. You might be able to squeeze 20psi out of it.
If you do use low-impedance injectors, you will need a resistor pack.
What are you doing for an intercooler? exhaust?