GOOSEer ... might I suggest 413,power ?

Here's my '92 D-350 hauler. Most good conversions involve a 4-foot frame stretch.

What's the max gvw on the door tag?

I have a low mile 78 440 and a fresh set of 67 big valve 915 heads to bump up the compression. Should be a good combo. Later idea is a late model 6.7 and a 6 speed manual trans.

A stock 78 440 is a weak combo ... if you go that direction get the block sonic checked

no NO ... the later model blocks are FAMOUS for being thin casting .., and those 67 heads ? ... under a heavy load you can TORCH EM !

10k bit the suspension will be way heavier than stock.

Ultimately it does not matter what the suspension is capable of... what will matter with the DOT in (you pick the state) is what the door tag says, That is why you see fewer and fewer (pick up based) ramp trucks. Here in the land of Taxes its approx $150 for the first 1-1000 Lbs of over weight,and goes up from there. Not including the impound, towing and all the other assessments they make once they find the first "cause" The second axle will need to have brakes equal to the first axle. BTW these are not NYS DOT regs but FED regs. If you dont plan on leaving you local area you may be fine but once you get out into the bustling metropolis its game on.
I used to have a Ramp truck (late '80s) 10000lbs door tag but 13000lbs going down the road loaded. Nice well maintained truck and could carry a Bulldozer (on more than on occasion) but stopping was another issue.
Those days are behind us...

None of this matters as it will be a Not For Hire project. I'm very much up on the knowledge needed for such a project. It will be way over built as with all of my truck and trailer projects. If I were to de a tandem the dead axle would be equipped with electric brakes. I hate to see units going down the road knowing they can't handle what they are towing or hauling. I'm a long time cdl-a driver, wreckmaster certified in towing and recovery as well as being an ASE certified tech in auto and trailer service and repair. I have built several trailers in the past. This truck will also tow a trailer. Trailer will have 3 3500lb axles as well as brakes on all 3 axles. I always use at least a 4 point tie down with anything on my trailers. The ramp truck will be no different. Actually I will use more when strapping down a vehicle on the ramp truck simply because I don't want something running over the cab with me in it. I will build a truck with the capability to do the job safely. And also being non commercial you can through any over weight tickets out the window as long as I'm under 26001 lbs.