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I have a particular liking for the IG Leman Russ tank and was wondering if I could substitue them into my BA army for a SM tank?

Normally no...

But, (!) you could use a second detachment to field it (you would need two troops and an HQ though). See P.87 under multiple detachments for more info.

Edit: Oh you could always ask who your playing if your friendly with them.

If your looking for counts as, you'll be hard pushed to find a match IMO. IG tanks are kinda the opposite to BA tanks, having a special rules making them slower than normal tanks while BA have faster than normal tanks.

I would have to agree. unless you're playing an allied list it's hard to legitimize.
IF your opponent is cool with you just straight up taking the tank in your BA list, right on!

or do some good conversions, like replace the turret with twin linked lascannons and give the sponsons lascannons - and you have a pred!

I would say no if I was playing you, but I wouldn't say no if you had converted it into something from the BA 'dex. If you converted it into something like a Baal Predator, I don't see why they wouldn't let you play it.

The Leman Russ has the Exterminator Variant which mounts a twin-linked autocannon, which is the same armament as the Annhilator Predator. Just nix the hull weapon and you should conform to weapons loadout. Alternatively field a Leman Russ Demolisher as a vindicator, with a large shield and just dont rotate the turret. Closest idea I can think of to field them.

Short answer no...most opponents would most likely disagree to you using a LR in a BA army, they don't need to be even more powerful!
But realistically you could ask you opponent if it would be ok if you split your forces into two, using say...750 of IG and 750 of BA, of course following all the rules for FOS.