Further HEI discussion

[69-Chevy-350]

I beg your forgiveness in advance but I have restored other vehicles, mostly Late 70's and ealry 80's BMWs. I need a little education about Hei. What it is and such. What parts I should put in and some general help. My digital cam is having technical problems so i can not show you my engine bay but I can describe what I am running into. The truck is a 69. The title says it and the data plate on the door jam says it. But the stock air filter and spark plugs for the 350 V-8 do not fit and I have had to use different size parts for that. The cap and rotor fit fine. However I am not sure if that was luck or what. I have the coil that seems to be for points with the middle piece being for the plug wires and the two side terminals for the different wires. What would you all recommend to do as an upgrade. I am not made of money so something not to expensive. I just want some better performance and increased mileage possibly.
Secondly, there is oil leaking from what apperas to be the oxygen sensor but i don't think it is or that these trucks even have them. It is plugged into the transmission (near the middle) and there is where it leaks. any ideas? Thanks guys for your assitance and help. I want to get this beast healthy.

Steve
69 Chevy 350 fleetside
81 BMW 528i