I'd say, firstly add the sponson weapons to the models for the lascannons. Perhaps you could the 30k barely present rounded sponsons instead of the regular
40k sponsons if you had the money and inclination to make them more thematic-looking.
https://www.forgeworld.co.uk/en-SE/Land-Raider-Proteus
Secondly you should include the front loading ramp from the
LR and look at placing he heavy bolters where those small guns are on the front of the vehicle. If that's not possible then place them in a small turret just above the drivers vision slit on the roof of the model.
Thirdly, remove the long main turret guns and stick em in your bits box. You can attach a combi-weapon to the front of the turret, where the targeting/vision apparatus are in the centre front of the turret.
Fourthly mount the vehicle high enough to match the
LR superior height so you don't get any benefit from the lower profile.
Doing these would place the weapons in near enough the correct places and would give no advantage over the regular
LR placement. All you'd be left with is the different design aesthetic.
This idea that models should look almost alike to what they represent is
BS. I've seen a Starwars
IG army where nothing looks anything like
GW stuff, but it was tournament legal. I've seen Squat armies that look nothing like
IG but they looked fine. The important factors are that models don't gain any advantage for being modelled differently (so you should stick as close as possible to the size, shape and general placement of weapons and critical features) and that they maintain some sort of over-arcing design aesthetic with the rest of your army.
If you're running Custodes and you don't mind converting/proxying all your vehicles for Forgeworld Custodes stuff then go for it. But things get a little weird if you roll with one of these proxied
LR's alongside an actual
LR model. Keep the aesthetic unified and you can get away with a lot more creative license.