I'm sorry if any of this seems elementary.
1. For the Crusader, don't glue the hurricane bolters together until after you've painted them. It's nigh impossible to get in between them when they're together, especially if you like detail.
2. If you don't want to glue the front door shut, be very, very careful. I was not.
3. Glue the side sponsons to the forward-most hatch. This will give you about an inch or so for measuring distances.
4. Don't limit yourself to the Crusader or the Redeemer. There are two ways to go about this.
a. Put a pin that is the same size as your drill bit in the piece of armour that fits on the outer portions of the sponson guns. Then drill a similarly sized hole where that piece attaches to the gun. Only glue the pin to the piece of armour. You can then press fit the armour and the gun. Repeat the same process for connecting the gun to the rotating column and the extra bit that fits on the opposite side of said column. This will allow you to change out the Crusader and Redeemer bits with only minimal pain.
b. You can also purchase one of the new Crusader/Redeemer sprues and simply build separate sponsons for each variant. Magnets can then be used to make them interchangeable. This is the better, if more expensive, option.
5. Use magnets on the sponsons. Go to Hobby Lobby and buy the large, I believe, rare earth metal magnets. They're about the size of an old brass thumbtack. Glue one on the inside of the LR's body on the backside of the sponson hatch. Glue the other on the backside of the sponson piece to which the guns attach. The gap between the backside of the sponson and the front side of the inner hatch is just right. Then you can pop the sponsons on and off to change them or when you get a weapon destroyed result. Lastly, don't put the sponson on the LR until the glue holding the magnets is COMPLETELY dried. The magnets are quite strong and will pull away from the plastic quite easily if the glue isn't finished drying.
Hope that helps,
Monkey