Phillips Valve PA

You're talking about the output transformer, not the power tranny.

Hard to tell what you might need to change without studying a schematic (or the amp itself) to see what's in there. I'm sure it's doable. For simply changing 75 ohm to 8 ohm impedance, simply adding a 3:1 stepdown transformer of appropriate volt-ampere rating between the 75 ohm output and the speaker(s) would be about right. That would be one way. To replace the output transformer, you'd need to know the nominal impedance the tubes need to "see" in the primary, and the output impedance. The square root of the ratio of the impedances is the turns ratio you need. That's how I just pulled a 3:1 stepdown out of the air for converting 75 ohms to 8 ohms. It's a little less than 10:1 impedance ratio. 9 is the square of 3. Close enough, impedances are approximate. In practice, an output transformer recommended for use with the tubes found in the amp of appropriate power rating will likely work fine without all the calculations. They come rated for a few popular primary impedances with the usual common output impedances. Guitar amps don't need to be hi-fi. Power transformers can work surprisingly well.

Ricochet has handled the output side nicely, so I'll do a bit of random thinking about the other end.

I'm going to assume this is the sort of amp that has a few inputs, and what is essentially a built-in mixer to control them.
Something similar to this.

If there is a "High-Z" (high impedance) mic input, that'll be your best choice. Your guitar's output should be a good impedance match, and your pickups will probably send a bit hotter signal to the amp than a mic would. Not so much that the volume control can't tame it down, though.
Don't bother with any "phono" input the amp might have. The level may be close, but it has a de-emphasis filter built in, which will make your guitar sound horrible. Of course you could remove that filter if you needed an extra input.

What tubes does it use?
If the mic pre-amp tubes are a similar type to 12AX7 it should have a fairly guitar amp-like pre-amp sound.

You might have to add a tone circuit of some sort, if the amp doesn't have any sort of EQ (or even a bass/treble control)
You might want to compare your amp's schematic to some of these well known guitar amps. You might find something that is quite similar, and might suggest some modifications.