RB Barrels? How do I link the right barrel to kit?

From  the description of kit and barrel,it sounds right, although not designed for that specific kit,it says generic,perhaps some surgery may be necessary ?

You could check with other vendors to see if they make a barrel just for your kit.  Have you checked in Scalemates?

McBean

A lot of the RB barrels are generic. I do like their barrels, but they are hard to get in the UK. A lot of their items are out of stock on their website, someone on Britmodeller did suggest they are going out of business, but there is nothing of their site to indicate it.

I would opt for 35B57, and its in stock on Models for sale.

https://www.modelsforsale.com/catalog/product_info.php?products_id=478135-RBM-1/35-35B57-Gun-Barrel-7.5cm-PaK40-L/46-(early-model)-Anti-tank-gun&utm_source=scalemates&utm_medium=cpc&utm_campaign=marketplace