Camper,
You will find the following section better suited for your Van;
1968 - Present Full Size Vans
Econolines are vastly different... the model years of 1967/earlier basically have nothing in common with those manufactured from model year 1968/later.
However, we'll try and help...
ENGINE:
Ford 302 = Ford 5.0, as does a 351 = 5.8... the math might not be exact, but that is how it plays out. As an aside, it is not uncommon to find Econolines in the U.S. with 460's in them... yeah, that's 7.5 Liters of fun.
BODY/ CHASSIS:
Ford Econolines are constructed of a separate Body and Chassis, meaning, yes it has a frame onto which the sheet metal attaches. They are NOT uni-body construction.
RIGHT-HAND DRIVE (RHD) OPTION:
I know of no U.S. Ford Econoline vans (note; these were basically built in only one facility) that were shipped to markets requiring such. A RHD conversion would be doable, since the front-end suspension is rather simple and rudimentary. But, finding the bits & pieces, along with the expertise is the biggest problem I can see. I suspect those Ford guys, who are experts with the [U.K.] Ford Transit's and Cargo's would be you best source.
** I suspect the major issue might be foot clearance... even after swapping out the dashboard/ heater, you must note that the Econoline has an engine that does not sit centered in the frame. In fact, the engine is biased towards the right-side of the van (likely done, since Ford engineers decided the right-hand side would be for passengers - and never intended/considered it to sell this in markets requiring drivers on that same side). **
You might also consider contacting some of the hotrodder's on your side of the pond... if there are guys into U.S. cars, and they are modifying them, I will bet a fair share have already done RHD modifications.
In the end, it comes down to engineering and dollars (or, Euro's)... it can be done, how much have you got?
Good luck...
p.s. (Knebworth Park/ 1979... Led Zeppelin was amazing)
p.p.s. ('Top Gear' is the best program on television, period!)
Trivia; 2013 is the end of the road for the Ford Econoline as we have known it... model year 2014 will bring about a very European inspired/ designed/ named Transit!