Also, can I put a sata ssd/m. 2 ssd in there 

Certainly you may install M.2 pcie ssd in 3070.

regarding psu upgrade you need 24 to 6 pin adapter since Dell motherboard uses proprietary 6 pin.

also Dell psu is TFX format, so your standard ATX psu does not fit in case.  ATX psu has to be installed loose outside case.  You might be able to install SFX psu in case though but you will need to carefully consider whether it fits.

What are the psu dimensions, and from where can I get the adapter? Is there something else 

Dell TFX Psu dimensions 85x65x225 mm

SFX psu 100 -125 x 125 x 63.5 mm, Standard Profile 63.5mm x 125mm

ATX psu 150 mm width, 86 mm height, and typically 140 mm depth

Adapter from Amazon https://www.amazon.com/COMeap-Power-Adapter-Motherboard-13-3-inch/dp/B08M5BZ6P8

What's the cheapest psu that fits the dell? 

@redxps630 

The comeap adapter is not valid for 3070 or any other dell model I know of and it lumps all dell models in together then says its not compatable with dell after saying it is. It also lumps in 3650 which is 8 pin as well as optiplex 7080.  If that were true you could upgrade a 3070 5070 7070 with a 3080 5080 7080 power supply.  HOWEVER this is not the case.  The G5 /XPS 8940/ optiplex 7080 power supply is not compatable with previous dell models. Its proprietary and its NOT ATX or TFX or any other standard.  There is no such  thing as 6 pin or 8 pin ATX connector for the motherboard.

Moddiy doesnt make an adapter for the 3070 or 3080.

What is the maximum length of gpu can the dell 3070 MT support

the optiplex 3070/7070 mt is one of the series 5 optiplex pc.  the 3070/7070 tower chassis looks remarkably similar to 3040 of the same series.  you may look on line to see what people did to the older 3040 case to upgrade gpu and get an idea of chassis physical limitations.  the 3 dimensions of 3070/7070 is nearly identical to 3040.

for example this video you can install a full length gpu double slot width about as long as the width of oem motherboard in 3040 or a bit longer (max length approaching length of chassis inside dimension close to 10.8"), likely same estimate for 3070/7070.

3070/7070mt: 

Height: 35 cm (13.8") x Width: 15.4 cm (6.1") x Depth: 27.4 cm (10.8")

3040: