A Pete 379 would make a cool conversion.
I can't tell you one way or the other about removing the sleeper, pro/cons pro- it adds a sleek aero transfer from the cab to the box, cons, you loose a few valuable sq ft particularly if you are thinking a lounge slide out.
As for build it yourself or farm it out?
Farm it out!! to someone like Hawk, SpaceCraft or a few others they have the expertise to stretch the truck, design in the ft/rear balance and build leak free slide outs. Once the box is built, wired, plumbed, and insulated you can then install the cabinets, flooring and wallboard. As well as trim and soft goods like curtains, interior doors, beds and sofa sleepers.
I would not attempt to build one from scratch for 2 reasons.
1) it takes too much time, a to learn everything you need to know just to design the truck, then the time and shop space it takes to construct the truck is incredible. Room for all the parts, a high ceiling to work on the roof etc. When you are all done you will think that with all you learned by mistake that perhaps you should build a second one just to use all your new knowledge and newly learned skillz.
2) Money and resale. It will cost you twice what you originally budget. If you are thinking 30' figure 80-100 grand to start. 3,000 bucks a foot without slides. Hawk and Spacecraft can give you good solid bids to get it to my above stage. From there you can use conventional pricing for the finish. Shop Menards, Home Depot, Lowes etc. to figure how much your shower, sinks, beds, countertops cabinets and flooring will cost.
Before you start anything have those finish numbers to add to your Hawks bid, don't forget to add in the truck cost, then decide whether you want to spend a year building (finishing) your truck or just spend a weekend on Racing Junk to pick out a completed unit for a 40% off, truck short sale and be trucking by the next weekend.
I mentioned resale, by having a coach builder build your box you can at least sell your project completed or where you give up with a brand to attract a potential buyer.
Does this make sense?
I just type as thoughts come to me- I hope you understand. You can search my project here and follow the whole build over on my escapees post, to give you an idea of how my thoughts have been backed up by 2 years of rookie RV manufacturing experience learned the hard way.
Good luck, looking forward to seeing your layout plans.
-blizz