What could cause this? 360 problems ####update####

Did you soak or pump up the lifters before installation? If so they may have been holding the valves open. I'm not sure how that became such common practice but it's not the right way to install lifters. Leave the pushrods out for a while (day or 2) and periodically check to see all of the hydraulic plungers can be pushed down completely. Once they are "free" you can reinstall the valvetrain and try starting it again. Before you do that though (and after degree-ing the cam of course) rotate the engine by hand; if it gets hard to turn STOP and check the lifters again to make sure none are pumped up. Then prime the oil pump while rotating it by hand again and make sure it turns smoothly through 2+ full rotations of the crankshaft.

When I put together my 360 that has a stock 5.9L Magnum short block (no valve reliefs in stock pistons), Edelbrock LA-based heads and a ~.550" lift hydraulic roller cam I had to be careful as some of the factory Magnum roller lifters I reused had somehow pumped up after sitting and those lifters prefer a lot of preload, like .080"-.100" iirc so with the correct pushrod length if they were fully pumped up that would cause the valves to be open an extra .100" or more due to rocker ratio. The piston-to-valve clearance was already on the tight side so when I first went to rotate the engine by hand some of the valves would start to contact the pistons and I'd have to let it then sit and bleed down the lifters, rotate some more, let them bleed down, repeat a few times and then finally they were all where they should be and I could start the engine.

If the lifters don't bleed down soon enough or you want to get it going sooner you can take each lifter out one at a time (so you don't mix them up), take them apart to drain oil out of the plungers and then reinstall. Hydraulic lifters of any kind should not see pressurized oil until they're installed in the engine with the rest of the valvetrain. Better to have it clatter and stumble a bit on initial startup as they pump up than have them hang the valves open.