The pipe thread is 3/8" NPT. I needed a 3/8" male to female 45 degree adapter in NPT for mine so I had to figure it out. I needed it for exhaust crossover clearance. This will at least help you in the aftermarket department. I would suggest using an electric one not mechanical though either one wil work.
I'm not sure either about aftermarket gauges but an 88 fiero sender will screw into the 4.9 oil filter housing and plug into the caddy wiring harness. You just use the stock fiero gauges. The earlier fiero senders will work but they are bigger and you need to change out the plug also. Good luck with whatever you decide.
[This message has been edited by Fieroseverywhere (edited 01-27-2008).]