The 4FCWX is rectangle. Mine is square. Will it still fit?

sorry I confused your pc with another model.  Actually it is easier in your case to use a standard form factor 460w-500w ATX psu (which has a black 6 pin PCIe connector) plus a 24 to 8 pin adapter for Dell inspiron 3670.

for the standard atx form factor you can use Dell xps/aurora 460w, or third party one.  currently best deal on market for new psu is EVGA for example 500W for 29.99

https://www.evga.com/products/product.aspx?pn=100-BA-0500-K1

Inspiron 3670 and 3650 use Dell thin form factor psu with propietary 8 pin (white) for motherboard.  I believe the 8 pin out is the same as Dell optiplex XE2.  there is a Dell psu for XE2 that has a PCIe 6pin connector (black) that you can use for GTX 1650.

Dell 4FCWX 

nvidia spec for gtx 1650 does not mention a 6 pin.  some card maker may have added a 6 pin.

Thanks. Can you recommend one that's silver?

Will the Gigabyte GeForce® GTX 1650 WINDFORCE fit in my Inspiron 3670 case or do I need one of their smaller cards? If so which one?

The Dell 365w gold psu 7vk45 has a six pin PCIe connector.  This is sufficient to drive a GTX 1650 in the inspiron 3670 based on psu calculator.

Load Wattage:244 W
Recommended PSU Wattage:294 W

This psu is designed for Inspiron 3670 as well as Optiplex 9020/7020 which also use Dell proprietary 8 pin white ATX 12V connector.  so this would be an easier upgrade of stock 290w psu (which has no 6 pin PCIe).

Thanks. This is what I'll go with. I'll just use this adapter. https://www.amazon.com/gp/product/B08P3FX9YG/ref=ox_sc_act_title_1?smid=A2276Y3SEPJUGD&psc=1

I installed the PSU and Windows wouldn't boot so I disabled Secure Boot. Now I can login but Windows restarts seconds after the loading the desktop. I restored my BIOS to Last Known Good Settings. This time windows loaded for about 5 minutes before restarting again.

The system works fine in safe mode.