I don't have any experience *building* cabs, but I've *listened* to quite a few. When I was shopping for guitar cabs I narrowed it down to wanting Celestion Vintage 30 speakers, so I tried all the makes that I could find that offered that speaker loaded into a 4x12. I tried the Mesa/Boogie, Marshall 1960AV and even a Peavey that had had aftermarket Vintage 30s put into it, among other boutique cabs. The cab construction made a HUGE difference, the Mesa sounded muddy to me, the Peavy just sounded like crap, and the Marshall was a very nice blend of all-around tone. As you can guess, I ended up with the Marshall.

Also I had a friend who built a 4x12 a few years ago. #1 the thing was about 175lbs -- ugh. #2 it sounded like GARBAGE. Not sure if it was the speakers, the electronics or the cab construction, but by the time he was done he'd spent something like $400 on the thing -- for that money you could get a used cab. He wasn't a dunce, either -- I think it's just a trial-and-error process.

So I think the physics of what's going on inside the cab makes a big difference. I'd hang on to the Marshall and re-speaker it if you have to. You might get less satisfaction than building it yourself but at least you know you won't be throwing good money after bad.

You might be able to remove the Marshal's front panel (where the speakers are mounted) and cut a new one to fit your 10's and 12's. This would be easier and cheaper than a total DIY cab and you'd know that the cab is well built and (i assume) good sounding.