Just installed rebuilt Pinto into mk2 van.  Started it up and oil light stays on. Shut it down immediately. Removed oil pressure sender and screwed in a pressure gauge, no pressure on idle, but jumps up if given a quick rev. 
Any suggestions?
Hate to think of having to remove the engine and strip again.

Hate to say it, but first thing that comes to mind is bearing clearances in the bottom end. Was the crank reground? Or loose oil pickup pipe maybe?
SFE

What grade of oil did you use?

Thanks guys     I used 15/40 oil    the crankshaft was checked by engine rebuild workshop   although I must admit that because the engine was completely original I didn't mic the shaft and installed std bearings   something to think about.   I was hoping somebody may have had an easy answer that would not mean pulling the motor out   Is it possible to drop the cross member to remove the sump?

Do you still have the old bearings, as normally they’re marked if oversize/undersize.

Yes will check (old bearings) tomorrow
 

In my opinion 15/40 is too thin for these older design engines.

Std bearings - took tappet cover off,   at idle almost no oil coming through onto cam.   
Any way of taking the sump off without pulling motor,   I have a grinder and welder

PeterM
In my opinion 15/40 is too thin for these older design engines.

I use penrite hpr30 in my 2000 ohc. If your low pressure is because of any increased clearances this could well fix it.

ronsmallford
I use penrite hpr30 in my 2000 ohc. If your low pressure is because of any increased clearances this could well fix it.

As do I. It has  the added bonus of a high amount of zinc in it to protect the cam followers.