My biggest issue is determining the difference between the 1986 H-body and the 1998 H-body sub frame dimensions, as well as tuning the PCM to work without the security system, etc.
Now on to the questions for anyone who has had experience with a similar swap...
-Do I have to change the length of the axles?
-I do have to fab some custom engine/trans mounts. In addition to the bottom 4 corner mounts on the '86, can I take one of those fancy wishbone mounts (like on the top front of a regal 3800) and brace the motor against the strut tower bar? Unless the torque axis mount is superior to 4 corners and the extra torque mount that is. 2 hours to change belts annoys me.
-Upgrade to the larger diameter brakes on the newer H-body? I already have 16" aluminum Buick wheels off a Regal GS.
-What size pulley before I have to change the exhaust and intake?
-Will the high flow cat. convertor really kill the low end and fuel economy?
-Does the trans have the HD 4th input shaft already?
-How much power can this 4t65e-(HD?) handle before it needs upgrades?
-Do I need any other wiring harnesses to make this easier?
-Dash swap from the 98 bonneville to the 86 or can I use my existing gauge cluster (tach, oil, temp, volts, speedo)
-Does the series 2 suffer from head gasket or L.I.M. problems?
My goal is 300HP, but I don't feel like doing any major mods until I'm satisfied that the setup will be reliable enough. I'm going to port & polish intake & exhaust and use a cold air intake though. I also plan to setup a sub-frame mock up and test stand to get everything working before I try to dump it in the car. Once in the car, I will be using the stand to try and find the problem with the original engine as well (it *must* not win).
That being said, anybody brave enough to take a gander at the original engines' problem I will have a few months before I do the swap, and I'm willing to test a lot of theories. If so, read on. If you don't care about an outdated engine and want to limit the topic to the L67 then you won't find the rest of my rant entertaining (well maybe entertaining).
And yes, it is a rant. 
I have a brand new original catalytic convertor manufactured in September of '86 (another failed theory BTW) and it didn't change a thing. I put the entire engine back to stock (replaced the high-flow intake I made with the stock one, replaced the exhaust system, etc.) and it hasn't changed a thing. It has a 4T60, TCC works, and if I put it in neutral it takes about 3 miles to coast from 60mph to a stop. The suspension is new last year. In my opinion it is a controls issue, specifically the spark is firing late. But the interupts for the crank sensor, and the crank sensor, are not adjustable so how could the timing be off? I adjusted the timing chain and sprocket alignment with the sensor according to the CSM (cam interupt triggers the sensor 20 degrees after TDC of piston #1, according to the service manual). According to the timing marks I made based on TDC piston #1, I have about 20 degrees spark advance at idle hot. It dips to exactly 0 degrees when the throttle is suddenly opened to WOT from idle. During normal driving, according to the ALDL datastream (almost useless at 160 Baud), cruising at 70mph netted me a whopping 58 degrees spark advance. I also have an issue with vapor lock. And no, I'm not guessing it's vapor lock. All the fuel in my fuel rail is vaporizing IF and only IF it's really hot out (above 90F), the engine is up to temp, and I shut it off and try to restart within about 15 minutes of shutoff. The vapor pressure doesn't release when the fuel pressure tap is opened, I have to physically disconnect the pressure line and relieve the pressure to get the pump to overcome the vapor pressure (new pump for the S/C engine and new fuel lines, etc so won't be an issue hopefully). I insulated all the fuel lines under the hood and nothing helps.
I do NOT have the original ECM or ICM. They were turned in for a core when we improperly diagnosed the problem when the timing chain jumped a tooth. I haven't been able to find an "X" cast in the block, and the sticker on the door says the car rolled off the assembly line in May of '86. I have checked and double checked that the cam sensor interrupt is properly positioned in relation to the camshaft and TDC of piston #1. I have 5 sets of ICM & ECM from other '86 and '87 Buicks with the same engine/trans, and every PROM available to each (so far 14 different ones). No matter the combination I have the same symptoms. I also had a '87 engine from a P.A. that did the exact same thing, but got 25mpg once during 100% ideal conditions which just happened to cause a rod bearing to spin. I have exhausted my resources and ability to diagnose the problem. I've asked everybody I can think of and nobody has come up with anything yet. I'm not giving up by swapping motors, but I have a better chance of figuring it out on a test stand than in an engine compartment...
Once I determine it's not at all fixable, I'm going to topswap a supercharger and see if I can get it to throw a rod. Should be fun.
