About to install new battery as Mechanic claims my battery is to small?

Found this info for reference:
1996 to 2004 Volvo S40 / V40 Battery Specifications
(max battery dimensions 278 x 175 x 190 mm)
Petrol Engines: 12v, 62Ah 520A H5 battery (optionally 72 Ah, 600A H6 battery[insert_php]$link_name =’bateria_H6_600A’; $FileLocation = “/inclds/insert_amz_ass.php”;include($_SERVER[“DOCUMENT_ROOT”].$FileLocation);[/insert_php])
Diesel Engines: 12v, 72Ah 600A H6 battery[insert_php]$link_name =’bateria_H6_600A’; $FileLocation = “/inclds/insert_amz_ass.php”;include($_SERVER[“DOCUMENT_ROOT”].$FileLocation);[/insert_php]

I'm putting in an Excide Extreme 12V 70AH 700CCA the biggest I've had in it to date!
Note the important numbers are the AH and RC..... so I'm actually 2AH short of Max specs 🤔

Mechanic said the clamp under the Coolant refill bottle is too be replaced as maybe cause of leak. Lifted it straight up out of position to check

The starter is located under that black plate in front of engine left of air intake pipe on right hang drive?

Battery installed... Took 2hrs as had to clean positive terminal and work out the remove the washer with paint on it.... that then loosened the terminals to remove.
Now pull up to remove battery locker and push down to install as per this video
https://youtu.be/-bARx9KIncU

From Rego Volvo said Sold as 2003 S40T.... but build is before Dec 2002... so is insurance not via the build date? It has the Grill and wheels of 2003.
Volvo Battery Part No. 30659797
$411.13 Could not tell me the CCA or AH. 😡

Now I found this on negative lead... repairable before replacing?
Volvo part No. 30857415
$125.84 Not available from Volvo anymore?

Regarding the leads: that will be the problem with starting - I'm surprised your mechanic hasn't found that. Best bet to replace is get a good one from a scrappie or take the original to an auto electrician and get a new one made up to pattern.

Also best to check the red cable going from the battery to the starter motor. If the cable is good, it still needs disconnecting at the starter and the terminals cleaning.

Appreciate the response.
I had more of a build up at the +ve terminal between battery and terminal than I could see before removed. I think the Red +ve cable has no issues at starter motor.
Will mention to mechanic if -ve needs replacing/repairing.

I have known corrosion to cause resistance where the battery cable is crimped in to the terminals, not easy to spot as it can't really be seen. May be a good idea to do a voltage drop test on the cables using a multimeter ?

What's the technique of voltage drop test on cables please?

Also I mentioned my coolant was low so I topped up but did not leave cap off on start to circulate, which I think has caused the bottom clip on hose to loosen and is now leaking.
Mechanic says next service will resolve but it's 6k away 🤦*♂️

To do a voltage drop test on battery cables. Set the multimeter on the lower settings volts DC.There are various points where you can test, but to start with, put the red probe on the positive battery post & the black one on the cable attachment stud on the starter motor & crank the engine.Ideally, the meter should give a zero voltage reading, but any voltage indicates some resistance, anything above 0.3 volt probably needs attention. You can also do this on the alternator terminal with the engine running to see if you lose any voltage whilst charging. To check the negative cable, reverse the cables & do a similar test from the battery negative post, & engine block & the body earth. You may need to disconnect the crank sensor to stop the engine firing up while you do the test, or you may not get a stable reading, also get someone to crank the engine, while you use the meter. There are various Youtube videos that probably demonstrate the procedure better than i can explain it

The radiator bottom hose clip is known for corroding & failing. replace it, preferably with a stainless steel clip.

Today after driving 20kms+, parking and then driving another 10km, parking returned to car after 5 minutes on flat ground..... went to start and all lights on dashboard and no turn over or sound in engine.
As per start of thread new battery.
Opened/closed door, glovebox and sat for a moment.
Eventually started as normal?
Ran without issue rest of day.

Cause?

So another two drives today and in another 7/11 all lights no noise.
Battery looked good and terminals.
Ended up putting key in opposite way.
Could the key/ignition be a factor?
Full tank of fuel too.