Can't seem to catch a break

What heads?
My first experience with an LA (318, In my case) was in an 88 D100. I went from great oil pressure to zip by changing the heads. That engine originally had '302s that turned out to be cracked (eng didn't run like cracked heads) and at the time I couldn't afford another set of them even from a junkyard. .
So I used a set of heads I had from an earlier 80s 318 just to get the thing running again. As it turns out the older head had smaller diameter pushrod holes and the flat tappet engine they came from had longer pushrods than the roller models did and had a different pushrod angle than the roller engine does and somehow I guess the roller pushrods need more room to pass thru the heads than the FT ones did ... I ended up getting money from dad and went to a yard and got me another set of '302s, bolted em up and my oil pressure came back....
I don't remember all the nitty gritty as that was back in the 90s.....