An uphill battle. . .literally

[miwise]

Hello,
I have been posting in this forum and have gotten some very useful help! I was hoping to get help with an engine problem now. I have a 1990 E150 Econoline Van with a 351 EFI and an E4OD Tranny. Just had A/C system repaired and the transmission rebuilt. The problem is that when going uphill in overdrive the engine struggles (but the transmission doesn't downshift). I think that the engine has a loss of power. Goes fine on level ground and great downhill. Maybe I'm just used to my car which has no trouble in these scenarios, but I didn't think that this van was underpowered with a 351 in it. It has a new catalytic converter, the air filter it good, and the spark plugs looked ok upon inspection. Checked the plug wires for corrosion and replaced the wire b/w the distributor and the transformer (I think it may be called a stator?). All other wires looked great based on my inspection (no corrosion and insulation good) and all were producing spark. I repaired a wire going to an injector as many of the wire strands were open leaving only a few for current to pass. Could not really tell a difference with any of these changes (except maybe the catalytic converter replacement). Tried running without the air cleaner, but same result. I replaced the air breather box. I did notice that the oil smells a bit like the exhaust, but no black smoke leaves the tail pipe. Perfectly clean (even upon initial startup). There is an oil pressure issue. It seems to have low oil pressure, so I tried switching oils. I went to 20w50 from 10w30. no change (except the pressure isn't quite as high as it used to be when I first start up). Pressure is good upon initial startup.

So, the reason for me being so detailed is b/c I thought that these issues may relate to the sluggish engine (like the cars on the commercial dragging the anchor among other things). The problem is worse when I pull my boat (it is a bayliner and I am not sure how much it weighs, but it is an 18 foot fiberglass runabout). The manual says to leave the vehicle in overdrive, but it doesn't have the power it needs to go up and down hills while pulling my boat in overdrive. Is this typical of these vans or is something wrong with the engine? I almost forgot. THere is a whistling sound every now and then when the engine revs and I'm moving down the road that I think might be a vacuum leak, but I don't know what effect that might have on the power in low rpms when going up and down hills in overdrive. Any help appreciated!!

Thanks!