Replacing smoothing capacitors in vintage guitar amplifier without datasheet
You measured the loudspeakers' DC resistance at 12 ohms each, which means they're "16 ohms" speakers. Note the speakers' impedance varies with frequency, it includes an inductive part and reactive parts due to mechanical resonances, cabinet, etc. So, it is normal you don't get 16 ohms on your multimeter for a "16 ohms" speaker. Here's a "8 ohms" speaker impedance vs frequency as an example (source):



There are two "16 ohms" speakers in parallel, so that makes it a "8 ohms" speaker.

If the amp is rated for "150W RMS into 8 ohms" then its peak output voltage will be around 49 volts. Allowing voltage headroom for the power transistors, this means the power supply voltage will be higher, which means 50V caps were a bad choice and it would be a very good idea to bump the voltage rating up to 63V.

If the amp isn't really "150W" supply voltage could be lower. Who knows.

You can't measure the supply voltage on the old caps with a voltmeter: electrolyte has leaked out, so they will no longer smooth the voltage properly, there will be a lot of ripple, and the voltmeter will not give the correct voltage. You can measure peak voltage with a scope though... or you could disconnect the transformer and measure its AC output voltage, and calculate the peak value... It's simpler and safer to just use 63V caps.

Note a "50V" cap won't pop as soon as it sees 51 Volts. It'll work fine at 55V, or even 60V, but its lifetime will be shortened drastically. If you use the amp 2 hours every weekend, hey, it will probably last through the 1 year warranty! 

Likewise, derating (like using a 63V cap at 55V) increases life time.

Now, the type of cap: either a snap-in or screw terminal cap. Snap-in is cheaper.

You can use 4700µF or more. I'd just pick one that fits in the mounting hardware you got. For a 150W amp I'd use more capacitance, like 6800 or 10000µF. Back in the day, when the amp was made, they were expensive, this is no longer the case.

Ripple current rating: at peak power (48V peak into 8R) is the amp is 150W, we'd have 6A peak if the loudspeaker was a resistive load. It is a reactive load though, so current will depend on frequency.

I'm sure you don't play at full power all the time. So you don't really need a cap with 6A ripple current. In fact, most likely any snap-in cap will do, they usually have 3-4A ripple current rating for this voltage and capacitance spec. It'll be fine.

If you want to feel good about yourself, you can use this one, it says "Disigned for high grade audio equipment, giving priority to high fidelity sound quality" ... (ie, pay extra for marketing). But really, any snap-in cap will do.

Note the wiring you got isn't optimal for hum reduction. Wires from the rectifier to the caps carry 100Hz current pulses, and wires from the cap to the amp carry a halfwave rectified version of the amp's output current. So it makes sense to minimize the loop area of these current paths to minimize radiated magnetic field and hum. So you can wire the input wire bundle tight together, and the same with the output wire bundle, like this crummy drawing: