Does the MC7805CT-BP need capacitors

1. Why use 5V if you don't need to?

2. 100 mA * (5-3.3) is 170 mW. No big deal for TO-92.

3. A cheapskate hobbyist approach is three silicon diodes in series. This will get you in the neighborhood of 3V. Might need to add a load resistor to set the max voltage with no card.

Have you thought about how you will level-shift the signal lines? SPI lines are all unidirectional, which should simplify things. 3.3V is a logic "high" for a 5V input. The outputs should be level-shifted somehow. A two-resistor divider would work ok. Use a 1.5k and 3.0 k resistor to get pretty close to 1000 ohms output impedance. You'll need this on clock, MOSI, chip select at least.

Related: How do I get an Arduino (5 V) and MPR121 (3.3 V) to talk?

As OP has not clarified what 6V depletable source is to be used, this answer predicates a battery which has a nominal full charge voltage of 6 Volts, and is operable down to 3 Volts.

As discussed in comments, there are few if any options for an integrated buck-boost device in a 0.1 inch pin pitch DIP form factor. Further, with most manufacturers DIP is rapidly going out of production to be replaced by SMD, so it is inadvisable to use a DIP part for a new design, even if one finds such an option.

A parametric search for SEPIC / buck-boost ICs with the stated criteria, on a major vendor site such as DigiKey yields several options in SOIC packages. These are leaded packages with a 0.05 inch pin pitch, not extremely difficult to hand-solder. One DDPAK-5

Examples:

• Linear Technology LT1572:

  1.25 Ampere monolithic switching regulator with inbuilt switching MOSFETs, in SOIC-16. Operates from 3 Volts to 30 Volts. Supports most switching topologies including SEPIC.

• On Semiconductor CS5171:

  1.5 Ampere switching regulator with inbuilt switching MOSFETs, in SOIC-8. Operates from 2.7 Volts to 30 Volts. Similar to above, supports several topologies including SEPIC.