LB vs SB, is one better than the other for towing?

[danheit]

Title states it all. I am going to eventually be towing an open car trailer with a 79 CJ7 on it. Probably somewhere in the neighborhood of 5000lbs. Going to be towing around 450 miles with the wife in the truck, and she does NOT like some of my sketchy "trips", so she will need to feel comfortable driving this herself.

Currently running a 71 C10 Longbed with a 350/350 and stock gearing (unsure at this time what the ratio is).

I am planning on swapping in a built TBI 350 and 700R4 that is on my garage floor.

Reason for asking, is that (and I know this sounds silly, but bear with me) when I do the swap, I will have to have the DS shortened. That's going to run me somewhere in the neighborhood of $200 for a cut and re-balance for the shaft, along with replacing the carrier bearing, which I would do since it's all apart anyways.

I can also go the route of picking up the shortbox frame that I have my eye on, which I can get for $200. I will then slide all of my power stuff under it, and use the swb driveshaft and rear axle from the 90 GMC that I took the engine and trans out of.

I will be around the same price either way, the work that it will take is no big deal, just time, and I will have a shortbox in the end. HOWEVER, if the long bed is more stable than the short bed, it would make more sense to just get the DS cut down. Sound right to you guys?

Opinions please! (and yes I know that swapping frames is a lot of work to avoid a custom driveshaft)