fuel should not be coming out of the vapour hose to the carb unless vacuum is applied to that hose as edward stated. that is a problem that needs to be addressed before proceeding. and i would take his advice on the regulator. fyi the impco model j can be replaced with impco cobra. they seem to be better quality.
i have had to wait overnight once for a flooded engine to clear. funny thing too it was a hyster xm.
if whoever is repairing this is unframillar with lpg systems it might be best to bring in someone licensed to assist.

check your instrument cluster and fuses there is also a delay relay box it will be located either under the air cleaner or under radiator recovery bottle there is a 1amp fuse remove it the delay relay controls spark also check your wiring harness plugs top of trans were it go up to engine it will rub against case and will short out and kill truck

turn off the fuel supply, and crank the truck for 30 seconds or so. if it fires, you have too much LPG going in. if it does not fire, try the trick of seeing if it will run on starting fluid or brake cleaner. if it does not even fire at all, you may not really have spark at the spark plug electrode.
you should only have fuel supply at the "carb" throttle when vac. pulls it in. you have a bad regulator if LPG is pouring out of the hose when it is disconnected.

He said he pulled the line off the carb, or what he said was the carb, and cranked the engine, said plenty of fuel was coming out.

You mention you are getting fuel out of the regulator, how is this being checked?

He checked all the fuses and all were good. Any other ideas?

Not sure if he checked the fuses, i'll have him try that if he hasn't already and let you know. Thanks.

check your fuses yet? this truck is not a C.A.R.B. truck (after 2003, computer controlled ignition, your truck is older).