I would spend the time, effort & money on upgrading the current rig rather than making the changes you describe. To move your chainplates out to the deck edge you'll probably have to build new uppers, and to add a second lower (assuming you only have one now - it's an 'in-line' lower?) you'll have to move the existing one either forward or aft. That's four new chainplates and two new uppers, possibly new spreaders, four new lowers - that's a hell of a lot of changes. As for the splicing, on a columbia 29 I think it would be rather out of place and so very much more expensive (either in your time or a professional's).

Have a rigger inspect your rig and walk you through your proposed changes - he/she may be able to point out more cost-efffective & performance enhancing changes that are not so drastic!

Best of luck.