I've got a 62 Buick with a 401 and a TH350 fitted on an adaptor plate and using 2 flexplates - the original nailhead one for the starter motor followed by a spacer then another flexplate to connect to the convertor in the TH350. As this second flexplate is cracked and the trans needs to come out I wanted to swap to a 200R4 as I've heard this is a straight forward swap but I do have a few questions:

do i need to use the 200R4 convertor or would a TH350 work?
what flexplate can I use - does the nailhead have the same crank bolt pattern as later BOP motors?
will a stock 200R4 live behind the nailhead - it's only a cruiser and i don't beat on it
are all 200R4s drilled for Chevy and BOP motors?
what is a good donor car at the junkyard for a 200R4 and are there any good/bad years?
I believe the trans crossmember location and driveshaft length and yoke are the same between the TH350 and 200R4?
do I need to hook up any electronics or is it just a tv cable and a 12v switched supply for the convertor lockup?

thanks  

I don't have all the answers but a few.

The TH350 converter will not work with the 200R4. If you do not use the lock up feature make sure to change the valvebody accordingly and have an ample trans cooler.
The Nailhead crank I am fairly certain is not the same bolt pattern as the BOP.
A healthy stock 200r4 from a performance car (G-bodies mostly) or a few select parts to beef up a 200r4 it should easily survive a mild 401.
All 200r4's are set up with both the Chevy and BOP pattern.
As for donor car, G-bodies are best, but any trans should be gone through.
Lengthwise, driveshaft should be the same, the cross member will have to be moved back a bit. The 200r4 is a slightly taller and longer trans than the TH350 so if clearance is tight now it may be non-existent afterwards.
The TV cable needs to be set appropriately and the electronics require either a kit that modulates the torque converter lock up or a modification to the valvebody to simply hook up a 12v source (I don't know how to do this but I've had it done on a 700r4).  

Thanks, any idea what flexplate has a nailhead crank bolt pattern and a th350 or th200r4 convertor pattern?  

I would think that taking a Nailhead flywheel and carefully centering a 200r4 converter on it and drilling new holes in the flywheel would work.  I did this for my Firebird when I put the 700r4 in it. I put neverseize on the torque converters bolt mounting base, used a transmission jack to seat the trans against the block (on the dowls) pushed the torque converter against the flywheel then carefully pulled it all apart, permanently marked the spots on the flywheel, removed it and drilled the holes.  The spacing of the bolts will automatically balance themselves out and for the power level I doubt it effects durability.  Worked great while I had the car.


I thought that the TH400 converter pattern would be the same as the 700r4's, but it was slightly off so I opt'ed to make new holes.  

Thanks again, I've since found out the crank adaptor takes a Chevy flexplate so I should just need to get a flexplate with the trans.  

No Problem, that should work perfectly. Flexplates are relatively cheap.  I have a question, how do you have the starter mounted? I was thinking that the original 401's had the starters mounted to the transmission housing, and the TH350 doesn't have a provision for this.  I'm just curious, if you have it mounted for the TH350 it will mount the same for the 200r4 or 700r4.  

the starter is mounted to the block and engages on the stock nailhead flexplate, then there is crank spacer which the th350 flexplate bolts to. I'm not sure what make the adaptor is  because it was on the car when i got it.  

im nearly positve that 200r4s came in chevy, BOP, and both bellhousing patterns so youll have to look for the right one. check some back issues of high performance pontiac from 2004 or 2005. they had a lengthy multi part series about installing the 200r4 into older cars. they covered EVERYTHING and even listed which 200r4 variants came in which cars and what years had minor differences.