Depends if the conversion was done recently or may years ago, most older converters cut out the valve and pasted over it. If its recent and you can feel it then maybe it was done correctly. The book (LH original) shows it on the right side behind the AC duct assembly, the hose, vac motor, bracket, actuating arm was screwed to the inside RH foot well, once uncrewed the door was removed twisting etc until it came into the foot well. The internal side cover panel which may have carpet Oh bugger ! that's the side shoud vacuum control - I think you want the Plenem Valve. It should be on the LH side - there is a duct in the way but it should be secured by 3-4 screws, vac hose should go thru the mounting bracket and loops around to the vac motor. But those scews should release it. Sorry to be vague, my car was converted way back in ~80 ish. Where it was thru the fire wall now has the steering column. I'd climb under the dash and have a look except the chickens and mice are all asleep.
